Real mileage of the car

[Linas.P]

Whilst discussing RC-F 0-60 speed based on speedo, I just had a thought... I know for a fact my speedo indicates 10% higher speed when on standard 17" wheels, which were on the car had for 160,000 miles, for last 22,000 I had 18"... I haven't specifically measured, but just based on rough calculation and difference in rear wheel circumference it should still be ~8% (higher indicated).

Now unless I am missing something, this means my car indicates 10% more on odometer as well... I mean I cannot find any reason why would speedometer would be incorrect and odometer would be miraculously correct?! So just funny thing to think about, but over 182,000 miles that makes 18,200 more mileage... not so tiny difference, I mean 10% is actually quite high difference when it comes to "semi" precision equipment like speedometer. 

[NemesisUK]

"While speedometers are calibrated to fudge the numbers a little, odometers are designed to reflect accurate mileage, Tetzlaff says."

"The odometer, which directly reflects what the sensors are recording, doesn't have the same built-in inaccuracy as the speedometer. If your speedometer is off, it doesn't necessarily follow that the odometer is inaccurate too."

https://www.theglobeandmail.com/globe-drive/culture/commuting/why-you-may-not-be-driving-as-fast-as-you-think/article11487709/

[Linas.P]

If we go by what article say then at least odometer is not specifically designed to show wrong distance, however it is not necessary correct either...

It is surprising that car would use 2 sources to derive same similar data, probably makes sense in older days when both were mechanical, but now I would expect ECU to report speed from some sensor (like crankshaft or even ABS wheel speed sensor) and use it for both odo and speedo. Obvious possibility - it does exactly that, but then adds margin on top for speedo and keeps odo without margins. 

I think I probably need to do some testing i.e. zero odometer drive for 100miles and see what I get... kind of curious.   

[KenMavor]

I think it is against the law for speedometers to read lower than the actual speed of the car......or something like that.  Imagine the furore if you were done for speeding and then found out that when the speedo registered 40mph you were actually doing 44/45mph in real numbers.

[Linas.P]

Yes exactly that - cannot read lower, but can read higher by 10% +4KM/H.

[Hadrian]

The Aussie "Auto Expert" has a Youtube video on this subject.

[NemesisUK]

3 hours ago, Hadrian said:

The Aussie "Auto Expert" has a Youtube video on this subject.

Link?

[Hadrian]

20 hours ago, NemesisUK said:

Link?

Although this applies specifically to Oz, it's pretty much universal. There are no doubt, lots of videos on this subject.

 

 

[Linas.P]

Yes... so nothing new here 110%+4km, but is odometer actually exactly 100% or is it 102% or 98%... that is why I am trying to find out.

[NemesisUK]

There's no designed in 'error' as with the speedometer. The odometer accurately counts the wheel revolutions, so the error will be the difference in circumference from new to worn tyres. Not easily calculated as it will depend on how the tyres wear, which depends on the style of driving and the type of roads driven...