Eli5 – If a car speedometer is inaccurate, is the odometer also inaccurate?

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I love how almost no one is even addressing the question, they’re all just pointing out that all speedometers are inaccurate and describing why. This is a difficult question because modern cars often do things in different ways than they used to, but generally speaking yes, the speedometer and odometer are driven by the same mechanism. Up until maybe the ‘90s or 2000s this was always a physical cable that leads from the transmission to the dashboard. There is a small output shaft on the transmission that spins a gear at a certain speed, which spins the cable and is measured by the speedometer. In theory you are supposed to replace the gear if you do something that would make the reading inaccurate, like changing the tire size. Anecdotally I have a car with a failing speedometer cable and when the speedometer isn’t spinning the odometer isn’t spinning either. Eventually these physical cables started to be replaced by an electronic sensor on the transmission that read the rotation of the gear and sent that information over wires to the speedometer, which had a motor in it to mimic how a cable-driven speedometer works. Again in these cars the speedometer and odometer are driven together, I had a car with a flaky electronic speedometer and once again the odometer would stop updating whenever the speedometer wasn’t working. But lately there have been other new changes. Digital odometers (and also digital speedometers) have replaced mechanical ones, as far as I’m aware they use exactly the same concept of having a sensor read a gear driven by the transmission but where that information goes and how it’s used is much more complex. But since the information is coming from the same place via the same mechanisms I don’t see any way it could be any more accurate or precise. So yes, with possible exceptions the speedometer and odometer are almost always driven by the same data so any discrepancy in one will be in the other as well. *** Edit: a lot of great points are being made in the replies, I guess I should emphasize that the newer and more high-tech a car is the harder this question is to answer. Nowadays there are so many different ways to do things and so much processing done to the raw speed data before it even reaches the speedometer or odometer. So there are certainly going to be exceptions to the way I described things working, that’s just traditionally the way it’s done and it doesn’t really make sense to take speed and odometer readings in different ways since that data is inherently correlated.

obi1kenobi1

If your speedometer is off because your wheels don’t have exactly the air pressure they’re supposed to have (or similar), then your odometer will also be off. However, your speedometer measures revolutions over time, so for a speedometer the time measurements can also be off (and for older cars that don’t use a digital speedometer the whole mechanism is by nature less accurate than the odometer).

fiendishrabbit

The speedometer is generally off by a percentage, so yours showing 70 when you’re actually doing 68 should show 35 when you’re actually doing 34. In the EU there is a law that says that a speedometer is not allowed to show a speed *below* your actual speed, and cannot show a speed more then 15% *above* the actual speed. Most cars I’ve owned have been out by around 6-8% — when I’m actually doing 100km/h they are showing 106 to 108km/h. (For this reason I tend to sent my cruise control to be about 8% higher than the speed limit). Manufacturer will add this error intentionally so that they definately do not show a speed *below* the actual speed — they would be a fined a very very large sum of money if that were to happen. As for the odometer, I can only surmise that that too will be inaccurate by the same amount. You can probably test this yourself — when driving at 60mph (by your speedometer), time yourself for 5 minutes and see if your odometer has moved on exactly 5 miles. Or 15 minutes and 15 miles. Use cruise control to maintain your speed (if you have it).

thetomahawk42

All speedometers are inaccurate. So, in order not to be sue, company always overestimate speed by 3%. So if you see 100 kph it’s really 97 kph (check with your phone gps app). The risk of getting a ticket is lower and companies don’t risk to be sue because the speedometer lied to you. And in the meantime, they save 3% on warranty because when your car just go over the guarantees mileage, let say at 100,000km, in reality your car only has 97,000km and is still under warranty.

WillingnessSouthern4

Lots of modern cars in the UK deliberately misreport the speed, telling you you are going faster than you are, by about 2mph. This is due to a law that makes an offence to report the speed as slower than you are actually going, and since the speedometer can’t be guaranteed to be 100% accurate this is a trick to prevent them illegally reporting the wrong speed.

MrRickSter