dsorensn

Searching around on this forum there have been a few posts on this topic,but as far as I could find no specific suggestions with regard to ways and means of recalibrating an A8's speedometer. Based on the GPS in my smart phone my car's speedo is a full 5 mph slow. Which is annoying, when you forget about the error and start driving normally you have the world passing you buy. Start going 5 over everywhere and then climb back in a car that has an accurate speedo you'll find your self explaining the source of your bad habits to the authorities.

Has anyone found a VAG control block that can be accessed to recalibrate the speedometer? There was a post on this last year and suggestion that there was, but no follow-up that I could find.

Don

Mike_k

Don, I don't think you're likely to find anything in the way of a calibration as the gauge should self-calibrate every time the vehicle is started. These are electronic gauges so they're receiving a signal from the PCM, telling it what speed to display.

The first thing I'd check if my speedo was off would be my tires. If you're not running stock tire sizes this will have an effect on the speedometer. I know a lot of people run skinnier winter tires and forget that the middle number on the tire size is not an indication of measurement but rather proportionate to the width of the tire. I.E. a 245 55 17 is physically taller than a 235 55 17.

This might all be obvious stuff that you've already canceled out though..

MP4.2+6.0

Manufacturers get fined if speedo's read low, but not if they read optimistically. Thus in general they red high. I have never seen/heard of a way to tune this on an Audi electronically.

Per the other post, you can approach it by changing tire size a bit. I found this same 5% error referenced more or less in a German test of the W12 that had a lot of data like this. Typical of the 27.0" tire sizes found stock w/ 18" and 19" sizes. The 20" fitment w/ 275/35 20's takes the diameter up to 27.7" and cleans up more than half that error. There are some other tire sizes that work with 18's and 19's that can have similar effects. Fllls out the wheel wells a bit too, which I like.

mmaguire2

Is there a way to tell the computer what size tires are on the car? I noticed this as well but in a different way. After switching tires the Estimated Arrival time in the navigation system is about 30-45 minutes wrong on long drives. If there is no way then it would seem to be an oversight on the part of Audi. I should be able to tell the car what tires on the car since this seems to effect a few systems.

dsorensn

Bummer. I was hoping there'd be a software based solution to this problem.

Both sets of tires I have for the car, nearly new summer and brand new winter, are what I understand to be the standard size for 19" rims, 225/40ZR-19. Looks like I'm going to have to put up with the problem until it's time to buy new shoes...

Don

Mike_k

And there's your problem. 255mm is the stock width which means that with the 225mm tire your overall wheel/ tire combo is shorter and thus your wheels are moving faster for any given speed when compared to a stock setup.

The correction would be in running the proper tire size or changing the gear ratio in the PCM which may or may not be achievable with a VAG COM.

dsorensn

Oops, typo'd, the tires are stock, went out in the garage and checked they are 255/40ZR-10. So is the spare for that matter.

Based on this calculator, http://www.paspeedo.com/calculator.htm, in the future I could up to a 255/50ZR - 19 and the speedo would be only 1 MPH over actual.

BTW, what does PCM stand for besides Pulse Code Modulation. Acronyms...

Brozee

My tires are 255 45 18. Michelin pilot sport a/s.
I've noticed my speedo is about 3 mph slower than my radar based radar detector displays.

Mister Bally

I've used stock size wheel/tire combinations and I notice that the "friendly your speed is" indications are 3 MPH off at 30 and 5 mph off at 50-60. The instrument panel always seems to read higher on German cars. Well, almost.

My '01 Beetle turbo only reads 2 MPH higher than actual at any speed. Every other VW or Audi read about 5 mph too fast.

Oh for the days of mechanical speedos and ratio adapters that were easily swapped out on the side of the transmission to correct this. However, note on your A8 using mile markers on your next long Interstate trip. The odometer seems to be right on for me with stock tires. Just the speedo reads generous.