Is satnav an accurate speedometer?

August 21, 2007 12:49 AM

Could the speedometer built into my car be less accurate than the speed assessed by my GPS satellite navigation?

I use TomTom satnav in my car (Smart Pulse) and the speed recorded by the actual speedometer is consistently 2-3mph faster than that recorded by the satnav. Initially I imagined this was just a function of the contour and camber of the road, so that the wheels were covering marginally more ground than the GPS signal would measure. However extensive investigation on very flat straight autobahn sections shows that the satnav maxes out at 84mph, while the car speedometer maxes out at 87mph (the engine is limited). Should I, or could I, get my car speedometer recalibrated?

posted by roofus to travel & transportation (15 comments total)

Yes: it can indeed be that inaccurate, and you can get it recalibrated.
posted by equalpants at 12:55 AM on August 21, 2007

Your car speedo is pretty much guaranteed to be less accurate than a GPS. GPS should be spot on (less than 1mph error), your car speedo only has to be from 0 to +10% to be road-worthy (depending on where you live obviously) and will typically over-read by a few mph.

I personally wouldn't bother with getting it recalibrated, but that's just me.
posted by markr at 12:58 AM on August 21, 2007

Speedometers are often off ~+/10% even with the OEM stock tires. If you've changed the size of your tires, they will be off even more. You can get your speedo re-calibrated, if you'd like (and it still will likely be just a little off from your GPS, just enough to drive you mad).
posted by jamaro at 1:02 AM on August 21, 2007

that should be +/-10%, above.
posted by jamaro at 1:04 AM on August 21, 2007

Car spedometers are more often calibrated fast than slow, so that people don't get speeding tickets and then get angry at the car manufacturer.
posted by aubilenon at 1:16 AM on August 21, 2007

The speed from a GPS receiver is actually more accurate than the position. The position relies on four or more satellites for a fix, and over short distances the positional error can be larger than the distance traveled. But the speed reading is based on phase differences between the signals from various satellites, which are much easier to measure accurately, even over short distances.

In an automotive application a GPS receiver makes a very accurate speedometer, though the low update rate makes it less than appropriate for drag racing or other vehicle dynamics studies. (There are high-performance GPS systems for these applications, though.)
posted by Myself at 1:26 AM on August 21, 2007

My speedometer is a few mph faster than true speed.
posted by Blazecock Pileon at 2:10 AM on August 21, 2007

Your speedometer isn't actually measuring your speed; it's measuring the rotation rate of your wheels. Anything that affects wheel diameter (like choice of tyre model, tyre inflation pressures, weight carried, wear on treads) is going to change the relationship between wheel rotation rate and car speed.

The more your tyres wear down, the more your speedo will tend to overstate your true speed, simply because the speedo has no way to find out that each turn of your wheels is taking you less far than it used to.

So, provided your speedo doesn't understate your speed when your tyres are new and pumped up as hard as you ever pump them, don't mess with it; it's accurate enough to stop you getting tickets if you pay attention to it, and that's really all it's for.
posted by flabdablet at 3:38 AM on August 21, 2007

nth-ing all the above. Plus:

Car manufacturers have an interest in setting your speedo so that it reads fast. If your speedo reads 10% over accurate then you are getting through your warranty miles faster too.

One of the big manufacturers (Honda, iirc) are currently fighting a court case about this in the USA.
posted by twine42 at 4:34 AM on August 21, 2007 [2 favorites]

Re: the Honda court case. At this point it isn't much of a fight; I'm waiting for my check.
posted by notyou at 7:24 AM on August 21, 2007

Yes, your car speedometer can be off that much. Motorcycle speedometers are even more wildly off. And it is almost always reading too fast.

This is one reason I suspect policemen will look at you with a suspicious eye when you say stuff like "I had it on cruise at 55mph, I swear!" when they catch you going 72. Most speedos are off, but they are GREATLY off in the direction of "too fast".

So, quite likely, your speedo was reading 75, or higher, when you got ticketed for 72, so you damn well knew you were going more than 55.
posted by Ynoxas at 11:53 AM on August 21, 2007

If your speedo reads 10% over accurate then you are getting through your warranty miles faster too.

Speedometers are not odometers.
posted by jamaro at 12:24 PM on August 21, 2007

"Speedometers are not odometers."

In many (most?) cars, even newer ones, speedometers and odometers are driven by the same sensor. Especially in cars that don't use computerized information to drive them, they are driven by the very same cable leading into the dash.

They both just count revolutions of a gear in the transmission. If the gear is spinning slightly faster than it should (giving the speedometer a high reading), then the odometer will accumulate miles faster as well.
posted by CrayDrygu at 12:47 PM on August 21, 2007

In many (most?) cars, even newer ones, speedometers and odometers are driven by the same sensor. Especially in cars that don't use computerized information to drive them, they are driven by the very same cable leading into the dash.

The odometer and speedometer on my motorcycle are driven by the same cable.

The speedometer reads about 10% fast, the odometer is dead on.

Just because they're driven from the same source doesn't mean they're calibrated the same way.
posted by flaterik at 3:11 PM on August 21, 2007

They read fast rather than slow because, in most places, that is the law. Your car cannot have a speedo that reads slow and be road-worthy. Wheel-driven speedometers are inherently inaccurate, and the law in most places says they are not allowed to read slow, so rather than try to get close and risk going under, they just make them a few percent fast. Obviously there is a bit of variation from car to car though.
posted by markr at 11:14 PM on August 22, 2007