Speedo Calibration

[Maj]

The 98 A4 Avant I recently bought came with 18" replica RS4 wheels fitted, which I think have cocked up the speedo calibration. When the speedo says I'm doing 70, my TomTom SatNav says I'm doing 63. The speedo is almost always 10% out with the satnav. Other cars I've used the satnav in have always been out with the speedo but not normally by this much.

I'm concerned that if the speedo is out by this much then the mileage will be out as well? The mileage is high as it is (168k), so is there a danger that its actually done a lot more than this?

If the speedo is out then is it fixable? Does it have to go back to Audi?

[Lex Luthor]

I think the speedo is processed to give an intentional error.
Use the trip computer average speed display, or the speed display in the climate control to see the true speed, then you will know how off it is.
Search this forum or google on how to display the speed in the CC.

[Seat til I die]

It cant possibly affect the mileage.
You could stick 22" wheels on there or 12" wheels it doesnt matter!!

The milometer runs off the gearbox turns, not the wheels turning on the road.

Ferris Bueller proved this spectacularly by reversing his mates dads Ferrari into their back garden!!

[prolfe]

It cant possibly affect the mileage.
You could stick 22" wheels on there or 12" wheels it doesnt matter!!

The milometer runs off the gearbox turns, not the wheels turning on the road.

Ferris Bueller proved this spectacularly by reversing his mates dads Ferrari into their back garden!!

My Nova 1.4 SR in reverse took the miles off.

[neil w]

By law speedo error can be up to 10%, you'll find that the speed readings on any sat navs is probably only 0.1 to 0.2 miles per hour out at any speed.
I've checked our Garmin and Tom Tom units against calibrated speedos and vascar and they've all been accurate.
As standard on my BMW bike the speedo runs 13mph fast at a genuine 70 mph (ie 83 mph indicated).
I wouldn't not worry about it rely on your sat nav speed reading.

[darkhorse1001]

It cant possibly affect the mileage.
You could stick 22" wheels on there or 12" wheels it doesnt matter!!

The milometer runs off the gearbox turns, not the wheels turning on the road.

Ferris Bueller proved this spectacularly by reversing his mates dads Ferrari into their back garden!!

surely you've just proved your point wrong.....


if the mileage is based on final driveshaft revolutions then the final wheel circumference is going to have a significant affect on the mileage calculation outcome.

ie, assuming a car has stock wheels with a rolling circumference of 1m the computer assumes every revolution of the final driveshaft moves the car 1m. stick larger aftermarket wheels on with a rolling cirumference of 1.5m on and the computer still thinks its travelling 1m for every driveshaft revolution where as in fact it is now travelling 1.5m. 1000 revolutions later and the distance is out by 1/2 a kilometre.

whether or not the car can recognise forward/backward motion and adds/subtracts inappropriately is another question.

[Boy]

Goodf Posting. I agree.

[darkhorse1001]

i think you can adjust the speedo reading using vagcom (but i may be wrong here, meaning reading the rest of this becomes irrelevant)

what i don't know is if it adjusts it in a variable or uniform manner.

ie uniform, does it in effect adjust what it takes as a zero reading meaning 20 becomes 30, 60 becomes 70, 100 becomes 110. (adding 10 to all readings) which would be of little use in this example problem, but maybe used to fix a dial misreading!?

or does it adjust by a variable/percentage, so 20 becomes 25, 40 becomes 50, 80 becomes 100... (all by an increase of 25&#37 which could solve misreadings due to increase/desrease in wheel circumference?

anyone?

[darkhorse1001]

incidently if you post your tyre dimensions on here someone will probably be able to tell you if they are larger/smaller circumference than stock, which is crucial.

[squinty]

two things;

firstly, under eu law the analogue speedo HAS to be set about 3 mph short of what the actual speed is. eg at 30 you are only actually travelling at 27. at 60, you are travelling at 57 etc....

secondly, changing wheel and tyre size WILL directly affect the speedo. use this to calculate the difference. you just need to know the sizing of the original tyre, and the new tyre. it will then calculate the % by which your speedo is affected.