I was wondering if i do a gear change will it change my spedo? I thought I read that the spedo was controled by the wheel sensors. Anybody know if this is true?

 

Speedometer is not effected by a gear change on the C5.

 

But tranny shift points may be (i think it depends on if you had 2:73 or 3:15 but who knows, maybe need adjusting for both)

 

Hmm, I have the 3.15 gears now. I was thinking of going with a 3.45

 

From forum member 'Mike Mercury' webpage about gear changes:

http://mikemercury.home.att.net/342.htm

1. Did I have to get my PCM reprogrammed for the new rear axle ratio?

There are many reasons to reprogram your PCM when performing a rear axle ratio change on the C5 A4 (manual trans C5's do not require any program changes). One of them is not... repeat... NOT speedometer error. The C5 does not suffer from speedo error when changing the rear axle ratio... PERIOD - END OF STORY.

The first reason to reprogram is to place the WOT shift points back where they will do the most good. By changing the rear axle ratio you will have moved the RPM's at which the PCM computer commands a shift during WOT (wide Open Throttle). The shift points will actually have climbed higher and you may hit the software rev limiter (6000rpm) before the PCM tells the tranny to shift. In this occurrence you never will actually get a 1 to 2 shift during WOT because the rev limiter will kick-in before the shift is ordered. Luckily I didn't have this problem. My 2002 LS1 shifted (before ratio change) at around 5600 rpm during WOT. The 3.42 gears raised that to 5900 rpm and I never hit the rev limiter. The 3.15 to 3.42 upgrade is the only ratio change where one might escape the PCM reprog.; depending on the year of the C5 and it's TC's amount of normal slippage. Even the 2.73 to 3.15 change is too great to avoid hitting the rev limiter before the A4 would shift.

The second reason to reprogram the PCM would be to avoid the "P1870" slip error-code (that eventually will turn on the check-engine light also). After the rear axle ratio change, the PCM thinks it detects a failing (slipping) torque converter because the RPM at the locked stage is higher than what it thinks should be (on the C5 A4 tranny, the locked mode of the factory TC is not a mechanical locking - but instead a higher fluid line pressure that eliminates practically all slippage in the TC). From what I've "heard" (but unable to verify) is that the 2001 and newer factory PCM software will allow 150 rpm of slippage before the PCM is commanded to set the P1870 fault code. The 3.15 to 3.42 upgrade is only an 8.6% change... apparently just under the wire. But, I have also read on the car forums that each torque converter (even the factory one) has it's own unique amount of slippage when locked. And if it slips a little more than normal (but still within spec) you could still set the P1870 error code and eventually get a check engine light. Any other ratio change - even the 2.73 to 3.15 - is well above the 150rpm allowable rate and you will need to have the PCM reprogrammed to stop the P1870 code; regardless of year . I was lucky (again) and did not cause any error codes which allowed me to skip PCM reprograming -WHEW-

After driving over a year this way - I sdecided to buy a programmer anyway; to maximize the shifting, firm up the shifts, and other changes.