I understand that modern cars have a sensor the measures the speed of the rotation of the tires and uses that to put out your speed on your speedometer. My car is about 2 mph off (so if I’m going exactly 70, I’ve used a couple different navigation systems that state I’m actually going 67-68), even though it has stock tires and rims and I’ve done no modifications. So if I drive for an hour, wouldn’t my car register that I’ve done 70 miles when I’ve actually done 67-68? Is there any compensation so that my odometer doesn’t go off by a couple thousand miles over its lifetime?
I understand that over the course of a couple hundred thousand miles a couple thousand extra may not matter, but I’m very curious.
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An odometer works by counting how many times the wheels of the car have rotated and multiplying it by the circumference of the wheels, it’s not related to the speedometer.
That said the calculation of the odometer won’t be correct if you change your tires for a different size
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.
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).
Many car manufacturers overestimate the speed displayed in order to reduce liability. The idea being to not have cars saying you are going the speed limit when you are actually exceeding it.
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.
Well, traditionally speedometer needles were magnetically-coupled to the spinning speedo cable while the odometer was directly driven. So if the speedo return spring was too strong or too weak, it would register inaccurately, independent of the odometer.
‘Course, that was years ago – now they’ve probably got metric lasers in there.
I am wondering what the test standard is. There must be laws saying what the accuracy must be for a company to say what the numbers are. Plus or minus five percent is probably the best to hope for.
Your speedometer is not “off”, it’s purposely showing you a few percent points more on your speed for obvious reasons: they want to err on the side of safety when the traffic signs tell you a MAX speed.
The error due to measurement inaccuracy is smaller than that.
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).
Yes. It is all fuzzy math. The idea of the odometer is to give the general idea of the use of a vehicle for non-experts. “Registering” your car and including the mileage also ensures that the odometer continues to increase over time and doesn’t stagnate or magically get rolled back. I learned that my non-stock wheels were causing me to drive faster than the speedometer showed. This helps with the mileage but doesn’t help with tickets.
As with any machine, precision is more important in some areas than others. In the engine, you need precision for the parts to last over time. The speedometer is important for school zones and police officers. The odometer is an approximate figure.
Why approximate? Because a car that drives 100K miles in New England, over winter, is going to show greater wear than one that’s driven 200K miles in Georgia as a commuter vehicle. The salt and snow will impact the core of the vehicle significantly. Long distance commuting in a mild-winter zone can keep a car looking really good. The mileage isn’t going to be as important as how those miles were applied.
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