A8cylinder

My 2004 A8 came with 20 inch Audi wheels, and 255/35-20 tires (Michelin Pilot Sports). I recently discovered that my speedometer is reading about 5Mph faster than my actual speed. (using a prrtable nav unit) So if I read 60 on the Speedo, I am only going 55... I took my car in to check out why, and the dealer told me this is the wrong wheel/tire combination for my car, and they could "calibrate" it for a mere $300.00. So... I paid my $78.00 diagnostic fee and said to forget it... since I do have 17" winter wheels/tires, which probably are "right" for the car??? - Does anyone understand this, or know how to calibrate the car???

knepster1

The audis are normally off 3 mph--3 mph less than what speedometer shows

Randy Welch

Though my RS6 seems to be about spot on.

Emauss

...seems that car manufacturers usually make speedos report over actual speed (...maybe only on Europe..BMW was mentioned specifically). The manufacturer is fined HEAVILY by the gov't if they under-report speed with their standard equipment, so they err on the other direction intentionally.
My BMW Motorcycle (R12C) reads about 8 MPH high!

MP4.2+6.0

And, I think your RS6 is correct because of the larger spec wheels--255/40 on 18's instead of the same spec in 17's for the 4.2. Or said another way, about an inch bigger diameter than standard C5 stock at about 26 inches, vs. 25. Use them on my 4.2 A6, so know that scenario.

MP4.2+6.0

My stock W12 wheels run 275/35 20. That has about a 27" diameter tire fitment. Stock tires in 19 (255/40) or 18 (255/45) typically run about 27" A german lanuage test report on the W12 running 18's (as I recall; might have been 19's) sowed a consistent 5% speedo error. The 275/35's clean up several percent of that, but also mean you can't fit a factory spare in the well--you get a 255/45 18 spare in that case.

I continue to think the 275/35 helps fill out the wheel well better, having also used 255's in 18's on a temp. basis getting my wheels refinished. Covert4.2 even went to a 285 recently if you read his posts.

On speedo calibration, I haven't ever heard this was possible on any Audi, electronic speedo or otherwise. The Nav can be calibrated for tire size, but I think that only matters for dead reckoning when the antenna can't find reception. On my C5 with D Navi I found it didn't really matter in everyday use where it almost always can receive sat signals.

Net, skip the calibration and consider sizing up/back to the std. 20" spec at 275, assuming you have the 9" width wheels in one of the std. Audi OEM spec choices.

MP4.2+6.0

Haven't seen a 2008 up close to confirm.

britinmn

Hate to bring up a dead thread - but I've just discovered I have the same problem ... and I just thought that everybody was driving extra fast to overtake me because I had an A8L, lol!!

Confirmed it at the weekend with a hand held GPS ... seems to be off around 4 mph at 70ish... i.e the speedo says 75 and I'm really going 71.. so right about 5% error as mentioned in other posts....

So a quick question:
1 - any opportunity to recalibrate via VAG-COM? This oculd be my great excuse to convince the wife that I NEED it Next time I'm near the dealership I'll take it in there and see what they say alos...

The good news is my 2004 A8L just turned 77K miles ... so it has really only done 73K ..... almost new again!

diebeste

Curiously, if you reset the average speed in the trip computer whilst maintaining a constant speed, you will notice that the trip computer actually knows your true speed... So does the 155mph (250km/h) limiter (don't ask)...

ludikraut

IIRC there is a VAG-COM hack that allows you to show your correct speed on the MFD in your gauge cluster. Now that I also own an A8, I'll have to see if it works. Once I have my VAG-COM, I'll try it out. I'll keep you guys posted.

l8r)