Speedo/mileage Question

[anon12]

Bit of a random one, but does anyone know how the mileage clock adds up, is it just based on the speedo's reading. If so is this not a big flaw, from my experience of comparing speedo to sat nav on a long straight road with no change in speed the speedo is about 5-10% higher than the sat nav (for my cinq, similar on parents focus). AFAIK under these circumstances the sat nav is the better measure. If so does this not mean that a mileage of 50,000 could actually be as low as 45000 for example?

Suppose it's not a big deal but on cars with 100K-200K miles there could be a big incosistancy.

 

[danhans115]

As far as I know it is from the same signal.

I know in some nissans they have a 'police mode' which is enabled using a diagnostic cable and some commands on the CAN network. Enabling this makes the car display a much more precise vehicle speed and also the speed in reverse. It maybe that the car knows its going say 37mph but displays 40mph to keep us on the safe side?

TBH a car with a lot of miles on it wont be worth much anyway so the difference in mileage doesnt really matter as it wont effect the resale value much.

 

[grimwau]

I can verify that the satnav speed is more accurate than a normal speedo. On my cars there is usually a 10% margin of error when comparing speeds (probably intentional as stated) but as soon as I use it in one of my lorries with a tacho it reads exactly the same as the speedo.

Perhaps people who regularly calculate their MPG based on odometer readings should also take note as their consumption figures will also be out by 10%.