The speedometer, a little rule-breaking bit of data visualization

November 4, 2015 10:40 AM

The question about why speedometers go so high has a number of good answers as to "why the top speeds exceed reality", but let's take a moment to enjoy the radial gauge's data visualization that breaks some standard data vis rules, and ponder the possibilities in its design.

If you're interested in the mechanics and history of the speedometer, Wikipedia has a ton of information and How Stuff Works has a seven page article on How Speedometers Work.

posted by filthy light thief (51 comments total)

Yeah, this touches on race car drivers rotating gauges so that a vertical needle means "normal," or a limit, or other at-a-glance meaning.
posted by rhizome at 10:54 AM on November 4, 2015 [1 favorite]

Umm..The Civic I drive has a digital speedometer and it makes so much more sense than the Murano I drive with analog. The Rule of Thumb is that a number will do just fine when quick changes in readout should not drive a reaction(a violent change in oil pressure will swing a needle, hopefully calling attention to itself. A violent change in speed will demand the operator's attention anywhere but the speedometer).

I'm pretty interested in going 3-6 mph over the speed limit, so a number is just great.
posted by Dmenet at 10:55 AM on November 4, 2015 [1 favorite]

But then Edsel. it had a speedo that looked more like that compass stuck to your dashboard.
posted by boilermonster at 10:55 AM on November 4, 2015 [1 favorite]

I have never seen a circular speedometer with a non-continuous scale like that (that I know of). Is that a common thing that I've just missed for mumble years?
posted by Etrigan at 10:56 AM on November 4, 2015

I wouldn't want a numeric readout of speed, because the number changes too much, the least significant digit would be flapping all the time, unless they purposefully smoothed it out and lied to me a bit. Small changes in the needle position aren't noticable. Flipping between 55 56 55 56 55 56 54 54 54 56 would be. (Although I might actually like a small graph where the right hand point was "current speed" and the points to the left were a history of recent speed. Like a bar graph, swept on the x-axis by time)
posted by RustyBrooks at 11:01 AM on November 4, 2015 [2 favorites]

Why do you even need to know how fast you're going? It's not useful if you're in traffic and are paying attention to driving safely with respect to others and the conditions, and it's not useful if you're on your own and paying attention to driving safely. No speed limit is above the safe speed for driving - unless conditions are bad, when knowing your exact speed shouldn't factor - and staying legal is something that's easier to arrange by having your car know the local limit. Some cars these days read speed limit signs through image recognition (better than drivers!), and there are umpty-zillion other ways to convey that information to your car's computers.
posted by Devonian at 11:03 AM on November 4, 2015 [1 favorite]

The old 120 max was completely fine. The newer 140-160 max actually makes it harder for me to read the speedometer at city speeds, because some hashmarks and labels have been removed. Yeah, I know that's not really relevant. Sorry. My new speedometer just annoys the hell out of me.
posted by yeolcoatl at 11:05 AM on November 4, 2015

I need to know how fast I'm going so I don't get a ticket for speeding.
posted by Dmenet at 11:07 AM on November 4, 2015 [4 favorites]

Analog works because you don't have to read numbers. Just a quick glance at the position of the needle and you know your speed. I find digital speedos too distracting.
posted by Thorzdad at 11:08 AM on November 4, 2015 [3 favorites]

No speed limit is above the safe speed for driving

Arguable where I live, but anyway the real issue is that the posted speed limit can easily be *below* what appears to be the safe limit for driving. On my way to work the speed drops from 45 to 30 for no discernable reason for a few blocks. I suspect it's because the density of houses crosses some magic number that makes it residential for those few blocks. There is no way you'd "know" you should have to slow down there.
posted by RustyBrooks at 11:09 AM on November 4, 2015 [2 favorites]

Why do you even need to know how fast you're going?

Humans are pretty notoriously bad at judging speed. Especially at higher speeds.

I can't tell you how many times I thought I was going 70 but I was really going 80. And there's a big safety difference between 70 and 80. Even bigger between 80 and 90.

...it's not useful if you're on your own and paying attention to driving safely

Humans are also really bad about assessing risk. A lot of places that seem like they would be safe to drive fast down but have a slow speed limit is because there's things like hidden drives or weird dips in the road.
posted by mayonnaises at 11:09 AM on November 4, 2015 [3 favorites]

The ideal speedometer is your speed plotted over the last five minutes on a log scale.
posted by mccarty.tim at 11:10 AM on November 4, 2015 [5 favorites]

Why do you even need to know how fast you're going?

Speed is an incredibly difficult thing for a human to judge without a dial. It's very dependent on the perception of your surroundings, where objects are located relative to your position, your height above the road, the width of the road... It's very easy to lose track of your speed. And it's a lot easier for a human to understand "taking a sharp corner over 25 mph is unsafe" versus "when the trees look like they're whipping by too fast, then it's unsafe".

Beyond that, how do you know where you are without knowing your speed? I know I started at noon from my house and drove on the highway for twenty minutes - where am I? How can you make any kind of dead reckoning judgments without knowing your speed?
posted by backseatpilot at 11:10 AM on November 4, 2015 [5 favorites]

When I was sixteen and learning to drive, my parents' minivan developed this problem where sometimes the speedometer would stop working and drift back down to zero. This was a known issue so I just sort of ignored it and kept up with traffic when it happened, but one time I was on the highway doing seventy-ish and even though I didn't think I was pushing the gas any harder I started speeding up! And I was doing seventy-five! And pushing the breaks but still going faster! And then eighty! And I was panicking! I couldn't slow down! Eighty five! More brake-pushing! And then...ninety? And I looked out the window and realized everyone was passing me and I was NOT going ninety, the needle had just never been more than halfway when the speedometer stopped working and this was a valuable lesson in paying some God damn attention to what was going on around me and using the gauges as a guideline and not objective truth.

I found out a year or so later that in fact if you were driving downhill the minivan WOULD go up to like 120 per the speedometer. My actual car (which I LOVED LOVED LOVED), an '85 Plymouth Chrysler Fifth Avenue, had a speedometer that topped out around eighty so I never had any idea how fast I was going. I'm better medicated now.
posted by Mrs. Pterodactyl at 11:11 AM on November 4, 2015 [11 favorites]

That Edsel photo up above reminds me of another design issue. The push button automatic transmission buttons in the steering wheel hub had an interesting problem. Too many real tight turns caused the wires behind the buttons to pull out.

As to speedometers I vote analog. I don't need accuracy.
posted by njohnson23 at 11:13 AM on November 4, 2015 [1 favorite]

The speedo doesn't help much, though, if you don't know what zone you're in. My speeding tickets have come not from me not knowing how fast I was going, but because I didn't know I'd gone from a 50 to a 30, and for missing a temporary sign on the motorway.

I didn't need a speedo. I needed a "TOO FAST!" alert, one that I didn't need to look at.
posted by Devonian at 11:14 AM on November 4, 2015

I can't tell you how many times I thought I was going 70 but I was really going 80. And there's a big safety difference between 70 and 80. Even bigger between 80 and 90.

You learn how good (or bad) you are at judging speed when the speed limit drops by increments of 10 every few miles, or when you're trying to maintain 75 mph while going up a steep incline and then down a longer decline. (Welcome to New Mexico! And watch out for the police - we can have around 10 adjacent jurisdictions in some stretches, with city, county, state, tribal and federal police of all stripes overlapping.)

Cruise control is a much-ignored modern marvel in both those situations. Really, it's overlooked in most situations outside of city traffic with frequent stops.
posted by filthy light thief at 11:14 AM on November 4, 2015 [1 favorite]

My mom has a Civic and the digital readout is fine for me. The one thing I don't like about it is the lag when accelerating onto the highway but it isn't really meaningful information anyway. It did cause me trouble once when I was driving it and I knew immediately that following the speedometer would make me drive too fast but couldn't figure out what was wrong. I had to drive by matching speed with those around me. Once we got to our destination I realized that my mom had probably toggled the display from km/h to mph by accident and it all made sense.
posted by any portmanteau in a storm at 11:16 AM on November 4, 2015

My first car had a digital instrument panel with a 7-segment display for the speedometer. I thought it looked cool as heck, but yeah, you couldn't just glance at it, you had to read it.

Also instead of a bunch of indicator lights it had a 14-segment text area captioned "System Scanner" that would say "DOOR AJAR" or "LOW BATT" or whatever else it wanted to cram into two rows of 4 letters each. Which was kind of cool as long as there was only one thing going wrong at any one time, which, well, I only paid $900 for that car.
posted by aubilenon at 11:17 AM on November 4, 2015 [2 favorites]

I didn't need a speedo. I needed a "TOO FAST!" alert

Your impressive meat brain excels at making decisions like that and is how we distinguish ourselves from the robots. "Am I going too fast" is a highly subjective judgment based on tons of external data that would require something bordering on emergent intelligence to make. Celebrate your meat brain powers!
posted by backseatpilot at 11:19 AM on November 4, 2015

I realized that my mom had probably toggled the display from km/h to mph by accident and it all made sense

Just wait until it gets reset to "knots" or "parsecs" (special Star Wars commemorative cars only).
posted by GenjiandProust at 11:22 AM on November 4, 2015

In relation to the humans are pretty notoriously bad at judging speed and km/MPH difficulties...

Late one night I flew into Boston Logan from Toronto, and Hertz was pretty much out of cars, but since I have good standing with them the rental guy asked if I'd like a little complementary upgrade to a Shelby GT500 they'd just gotten in.

So I'm driving to my hotel, a little past midnight on a deserted expressway, rumbling along quite nicely. Thinking to myself, "hey this is a smooth ride", glance down at the dash, "oh I'm only going 115", double take... MILES PER HOUR... Ease off the gas a bit...

Fun car, but yeah if it's not your car and there aren't a tonne of points of reference it's easy to have no idea how fast you really are going.
posted by cirhosis at 11:35 AM on November 4, 2015 [5 favorites]

I have an FR-S and it has a 143mph top speed with a 160mph speedometer. It can't be having highway speeds with a needle straight up because 80mph is below halfway on a left mounted speedometer.

I dunno. Like most things in the world I guess everyone is different and has different reasons for doing things the way they did it.
posted by Talez at 11:40 AM on November 4, 2015

Why do you even need to know

If you are riding a motorcycle, chances are that your body has an accurate kinetic sense of what your speed is--not necessarily in MPH, but in what you think you can do with the bike. In most cars your body's appreciation of your speed is about zero--you get no direct feedback. You can't usefully distinguish between 40 MPH and 80 MPH. So, the turn you are approaching may require you to slow to about 40, a thing you won't come to realize until you are doing a barrel roll over the guard rails.

I like color scales, though.

Needle positions give me enough accuracy, and a quicker read, so hooray for the radial dial. I like to evoke the tach function, even though both our cars have automatic transmission. In the past I found the tach useful when pulling horse trailers in the mountains; it's better to establish a speed relative to the grade, and hold it than it is to keep shifting up and down.

Our Jeep thoughtfully provides a couple of colored lines to show my average mpg, and current rate of fuel usage. On my Chevy, I have to click around on a menu to get this stuff, so it's not as entertaining.

Neat stuff. At traffic lights I get to look at my tire pressures.
posted by mule98J at 11:43 AM on November 4, 2015

For another take on the speedometer, trust Volvo to come up with something that is simultaneously innovative and useless.
posted by brokkr at 11:51 AM on November 4, 2015

njohnson23: That Edsel photo up above reminds me of another design issue. The push button automatic transmission buttons in the steering wheel hub had an interesting problem. Too many real tight turns caused the wires behind the buttons to pull out.

Sorry about that (by proxy). My grandfather designed those transmission linkages and controls (along with a bunch of other crazy crap in the Edsel). He explained it to me once, and it was supremely hairy. That it worked at all was a minor miracle.
posted by Xyanthilous P. Harrierstick at 11:59 AM on November 4, 2015 [5 favorites]

There are speedos that display 160 mph when the cars are limited to a leisurely 135 mph, and then there are cars that only register up to 85 when I know they can go faster.

My friend borrowed my early 1990s Buick LeSabre for some road trip that was related to a college class. But somehow, he ended up following some in another car at high speeds in the middle of the night. He told me that my car could go around 115 mph, but at that speed it guzzled gas like no other.
posted by filthy light thief at 12:16 PM on November 4, 2015

I've always liked the densely packed speedometer of my xB, tucked away from the steering wheel in a more central location. And the speed gauge goes to 120, which is eminently reasonable as I've been able to get up to 110 a few times when the road is very straight and the wind is favorable.
posted by FatherDagon at 12:38 PM on November 4, 2015

I've done a reasonable bit of driving through the outback in Australia and it's quite a weird experience to have been sitting on 130km/h for so many hours straight that you feel like you aren't moving. It becomes the new normal and yeah, without that dial you'd have no idea.

I remember being amazed the first time I drove up in the north of South Australia that I had left my foot in exactly the same position on the accelerator for three hours.
posted by deadwax at 12:45 PM on November 4, 2015 [3 favorites]

As a back seat driver, I liked the big LED speedometer displays.
posted by lagomorphius at 12:55 PM on November 4, 2015 [3 favorites]

Mostly you need to make sure you're above your stall speed and below your never exceed speed, no?
posted by Monday, stony Monday at 1:05 PM on November 4, 2015 [3 favorites]

Filthy light thief--there was a regulation in the USA for most of the 80s (longer? not sure) that required speedometers to have 85 mph as the highest marking, and with a more prominent marking for 55 mph.

This post makes me think of the '64 T-bird my dad had, with a thermometer-style speedo, and secondary gauges mounted in little individual pods. The whole effect was very Buck Rogers.
posted by adamrice at 1:56 PM on November 4, 2015

My first thought was a digital/numeric display, with color indicating acceleration. Bright red numbers would indicate high negative acceleration (i.e., harsh braking), pinkish numbers for gradual deceleration (e.g., coasting). Bright green numbers for sharp positive acceleration, etc. Of course, this runs into issues with colorblindness.

"The ideal speedometer is your speed plotted over the last five minutes on a log scale."

This gives me another idea: a virtual dial with needle history visualization. Meaning: Every tenth of a second or so, the (virtual) speedo needle leaves a sort of "echo" (emanation?) behind, which gradually fades over the course of a few seconds. So you have sort of a 5-second history of your speed. Which I suppose basically would look like a long-exposure shot of a regular speedo needle under comparable acceleration, except with better color/brightness differentiation (and the echo trail would never be more than half as bright as the needle itself).

I'm sure some more modern and high-end cars probably have something like this already. If not, I claim the patent!
posted by Eideteker at 2:03 PM on November 4, 2015 [1 favorite]

The digital speedo in the '83 Mazda we had gave several reads per second, but didn't interpolate the values between the read at, say, t1 and t2.

As a teenage nerd, this amused me, since the display jumping from 0 to 3 to 8 to 10 to 15 during acceleration from stops sort of implies that AT NO POINT were you going 6 miles an hour.

Since then, every car I've had has used the conventional needle versions, though.
posted by uberchet at 2:09 PM on November 4, 2015 [1 favorite]

the real issue is that the posted speed limit can easily be *below* what appears to be the safe limit for driving

Around here (Ontario) (and I suspect most other places too), it is by definition. "Excess Speed" is defined as more than 10km/h above the posted limit. Which is to say that the posted limit is not the actual limit.

A lot of that's probably just to account for slight speedo error, natural fluctuations in speed, and maybe a higher likelihood of contested tickets for speeds just slightly over the limit, but I'll be darned if the natural flow of traffic isn't always exactly ten over the limit.

Why do you even need to know how fast you're going?

If you're on a highway going 110km/h for a while, and you take an exit into a 50km/h zone, it is very hard to gauge your speed. The speed limit (posted or actual) will seem like a snail's pace. The human brain is extremely unreliable, and that's pretty much why all meters exist.
posted by Sys Rq at 2:28 PM on November 4, 2015

Why do you even need to know how fast you're going? It's not useful if you're in traffic and are paying attention to driving safely with respect to others and the conditions...

There is an automated speed trap at a school zone right near my house. On Monday-Friday between 6 AM and 6 PM, the speed limit is 20. Drive more than 25 in there, and you're going to get a very expensive ticket in the mail. The road is two lanes each way plus a turn lane in the middle, and there are no children or other pedestrians crossing except at specific times. 40-45 would be perfectly safe.
posted by Foosnark at 2:34 PM on November 4, 2015

On Monday-Friday between 6 AM and 6 PM, the speed limit is 20. Drive more than 25 in there, and you're going to get a very expensive ticket in the mail. The road is two lanes each way plus a turn lane in the middle, and there are no children or other pedestrians crossing except at specific times. 40-45 would be perfectly safe.

Here in suburban Boston it's 12:30pm-4:30pm for the 20 zone. Not a fucking kid in sight for miles because they're all in class and I'm technically supposed to be going 20.

It's not really an excuse because the law is the law but people just lose respect for it when the law is applied so broadly and stupidly.
posted by Talez at 2:44 PM on November 4, 2015

"Eighty percent-plus of the cars on the road are not designed for and will not go over 110 mph."

A dumb, reckless teenager can get a used 4-cylinder Honda Accord up to at least 120 mph. Don't ask me how I know that.
posted by dephlogisticated at 3:27 PM on November 4, 2015 [3 favorites]

A dumb, reckless teenager can get a used 4-cylinder Honda Accord up to at least 120 mph. Don't ask me how I know that.

I can tell you exactly why. Because an F22B1 makes 145hp at the flywheel, the car only weighs 2800lbs and air drag is a coefficient of the square of the relative flow speed.
posted by Talez at 3:37 PM on November 4, 2015 [6 favorites]

I don't think there's one right answer here. I had a Toyota Tercel in college that couldn't hit 100 and drive a car now that goes until the limiter kicks in at 155.
posted by Blue Meanie at 3:53 PM on November 4, 2015

The articles miss two things that are important IMO. The first is that we usually see the speedometer through the moving window of the steering wheel. You can't read a digital gauge if the leftmost digit is obscured; you can't read it precisely if any digit is obscured. You can't fix this by making the readout larger or smaller. A needle, on the other hand, can be read if any portion of it is visible. So that's one reason.

The other is, we are actually pretty great at recognising and correcting divergences from a desired angle. Drivers do this all day long with remarkable accuracy, even though there's no direct relationship between the angle of their steering wheels and the angle of their tires. The same goes for the speedometer: experience shows that it's pretty easy to use peripheral vision to keep the needle at a constant angle, or even (after a visual check) to change the angle by ten or fifteen degrees. In contrast, it would be really card to use peripheral vision to recognise when a digital display has dropped from 63 to 58.
posted by Joe in Australia at 4:59 PM on November 4, 2015 [2 favorites]

A dumb, reckless teenager can get a used 4-cylinder Honda Accord up to at least 120 mph. Don't ask me how I know that.

In high school, I drove the family station wagon, which was quite literally older than I -- my parents had bought it when my mother found out she was pregnant. It was a Ford LTD Country Squire, which my father claimed held the second-largest engine Ford ever put in a production vehicle. It was, of course, more than seventeen years old when I was driving it, and while my parents had kept it in reasonably good shape, it was still a Ford LTD Country Squire that was nearly old enough to vote.

So one day, driving home from school, I was waiting at a stoplight to get onto the freeway when some friends pulled up in a much smaller and newer car. We waved, and they revved their engine at me, to much laughter. I revved back, to even more laughter. They zoomed up the ramp onto the freeway, everyone still laughing.

Until about a mile and a half down the road, when I passed them doing well over 120 mph, the car shaking and shrieking like the Devil himself was pushing it. I was waiting for them at the next exit (the one we both needed to use to get home). They weren't laughing anymore.
posted by Etrigan at 5:48 PM on November 4, 2015 [4 favorites]

Why do you even need to know how fast you're going?

Two days ago I drove over some mountain passes in falling snow, in the dark. The swirling snowflakes in the headlights were extraordinarily disorienting, and without the speedometer it would have been hard to know if you were standing still or going 80.
posted by Dip Flash at 6:37 PM on November 4, 2015 [3 favorites]

Somewhere around Hermiston, OR, highway 84 descends from the mountain range to follow the Columbia river / gorge. There are signs reminding you how to slow down, because it's a very long, very straight stretch of road, and I'm pretty sure everyone's mom's shitty minivan can top out the speedometer, if placed in neutral.
posted by pwnguin at 7:01 PM on November 4, 2015

Humans are pretty notoriously bad at judging speed. Especially at higher speeds.

If everything in front of you looks a little more blue than you expect, you're probably going too fast.
posted by Harvey Kilobit at 7:20 PM on November 4, 2015 [4 favorites]

There's definitely a speed where I feel a little bit out of control - usually 10-15 miles above the speed limit. I have a leadfoot but try to rein it in when I think I can't handle it. For speed I rely on comfort level and I mostly watch the tach and try to keep that in a good range.
posted by bendy at 7:53 PM on November 4, 2015

The best thing about being able to set an exact speed with cruise control, I've discovered, is that it seems to be the only way to ride in highway bubbles for long periods of time. When highways are moderately empty, cars tend to clump up in periodic waves, and when you're using the gas pedal, it's almost impossible not to gradually catch up with the wave ahead of you even if you're not intending to. But if you engage cruise control at a hair under prevailing speed, pretty soon the clump leaves you behind; tap the cruise up by 1mph, and you can stay in a nearly empty bubble between the clumps for miles, which feels especially peaceful and deserted at night. For whatever reason, though, trying to do this manually is almost impossible, due perhaps to the natural tendency to gauge speed by the car in front of you or the subconscious desire to hurry and catch up with the people ahead of you.
posted by chortly at 9:44 PM on November 4, 2015 [2 favorites]

had a speedometer that topped out around eighty so I never had any idea how fast I was going

Riding around in my friend's Ford Taurus (station wagon!), we had a similar situation. We simply referred to everything above the last marking as "ludicrous speed." We spent quite a lot of time going ludicrous speed in that wagon.

My own car's speedo went up to a proper 120mph, but I placed my own hashmark at 88mph. That way I could know when I had reached a proper cruising speed of 1 delorean.
posted by Panjandrum at 12:18 AM on November 5, 2015

I used to drive a '66 plymouth belvedere. Gigantic battleship of a car, with an equally enormous engine that sounded pretty much satanic. The non-clutched direct drive radiator fan would "sing" in certain RPM ranges on top of the thundering V8 sound. You could get four people in the front, and four in the back... and still have more elbow room than the back seats of almost any new car. It was also the nicest car to just cruise on the highway in i've ever driven. It was loud, and the steering was tapioca... But it was so fucking smooth and the seats were amazingly comfortable. It felt like sailing, not driving.

The speedometer was also fucked. It would occasionally make a loud SNAP--TING and either spring back in to action or just... stick. It tended to stick at 70mph-ish. None of the other gauges really worked right either or made logical sense, but the speedometer and the gas gauge were the only actually annoying ones.

The problem was, that car would pull HARD until it ran out of gear and hit the redline. Which was, i don't know, probably closer to 150mph than not.

More than once, i'd be cruising down I5 just passing everyone and going "man, what the fuck is with people always driving 50mph on the highway around seattle" only to pull off and realizing i had gotten from olympia to seattle in like, 35 minutes.

Oops.

I didn't need a speedo. I needed a "TOO FAST!" alert

It doesn't handle temporary zones, but i always found GPS units that showed the current speed and the speed limit right on that patch of road really cool. One i saw would actually change the whole outline of the display bright yellow or red if you were "speeding".

GPS units are buggy whips now, but i always found that one function rather cool. I always kind of idly wondered if there was a smartphone maps app or even just a HUD one that did that. Even just

CURRENT SPEED: 61
SPEED LIMIT: 60

With a configurable "slop", and a background that changed from black to neon red or something would be super useful. I'm actually sort of amazed this isn't some function of the google maps app, now that i think about it.
posted by emptythought at 3:29 AM on November 5, 2015 [1 favorite]

The Nokia Here app will show your current speed and the speed limit. I've found it pretty useful, but the audible speeding warning sound gets annoying after a while.
posted by bradf at 9:36 AM on November 5, 2015

I wonder if it'd be useful for cars to tell you how fast you're moving compared to what's in front of you? A lot of accident stories start with "I thought the other car was driving at the speed limit but then suddenly..." Seems it'd be easy to implement with radar/lidar/etc showing up in other safety/self-parking features on luxury cars.
posted by mccarty.tim at 3:04 PM on November 5, 2015