Bill, my limited understanding of autos tells me that while what you wrote here is true...I think the fact that the speedometer is reading 10% high means that the car is shifting at an RPM 10% below where it's been programmed to shift. Under part throttle that's not enough to worry about, but for max performance, it would matter to some degree. At least it's not 10% high which would cause it to hit the rev limiter, as some auto guys do when they put on smaller drag radials.

My confidence in the statement above is not 100%. Perhaps someone with more automatic experience than me will chime in or perhaps you know Bill.

Perhaps also someone with a Tech 2 will chime in on if it's possible to reset the vss sensor with the Tech 2. I think it is possible but I don't know how. I know Mike Norris has a Tech 2 and would likely know if he sees this thread.

 

No solutions from me, but this is really weird. On both our 2006 and now the 2009, the analog speedometer was accurate within 1 mph at all speeds up to 150 mph. I didn't check it higher than that. The HUD would usually read 1 mph less than the analog.

This sure sounds like some kind or programming issue, unless the car has mods that haven't been mentioned.

 

Robert, does the car have stock tires? Aftermarket tires that are smaller will cause this problem, and if that's the case, you could buy new tires in stock height (you'll need front and rear to keep the ratio within limits or you'll get "active drag control" issues).

 

Im learning C6 by the minute! The VSS for an AUTOMATIC in in the trand tail shaft.

 

Uhhh....yeah it does and especially at WOT where it's the only thing dictating what the shift points are (which by the way I eventually figured out at the dragstrip after countless frustrating hours of not being able to figure out why the car wouldn't shift at the RPM I was telling it to).

Here'a a somewhat oversimplified example to illustrate my point:

For simplicity's sake let's say that one of the WOT shift points in an untouched US car happens at exactly 60MPH and 6000RPM. Now given that a discrepancy of 10% could noticeably alter the performance of the car, I'll bet money the OP's car would make that same shift at 66MPH/6000RPM since the 6MPH discrepancy is likely just what's being reported on the speedometer whereas the transmission still knows the car is still only going 60. Even if I'm wrong about this one fact it won't invalidate what the correct fix is.

Part one of the fix (which a few people already mentioned) would be to adjust the tire size in the PCM upwards until the MPH is reported accurately. There's no need to change the gear ratio however as that'll throw too many other things off.

The second part of the fix addresses the OP's concerns about inadvertently changing the shift points, which it will once you start altering the tire size. Using our example from earlier, once the tire size has been changed to get the speedometer reporting 60MPH again (instead of 66MPH) the resulting side effect will be a 60MPH/5400RPM shift point. In order to account for that you'd then need to bump up the MPH for the shift point in your tune until it's shifting at 6000RPM once again.

Now if you've been following along you might find yourself thinking "hey, that won't work since his car will be shifting at a higher MPH than before". While that may be true, the bottom line is that like a manual trans car the accuracy of the MPH at a given point doesn't matter because only the RPM is important.

To give you a real world example of this, my 1-2 shift is set to happen at 37MPH/7200RPM and despite my speedometer being off by about 3.4% (it'll read 62 when the car is only going 60) I really don't care if I'm actually doing 35, 37, 39 or something else entirely different just so long as my car shifts at the RPM I want it to. If I ever do want to alter the RPM of that shift, I would then just go into my tune and either raise or lower the WOT shift MPH to achieve the desired effect.

Now having said all of this, it might ultimately be easier to program the car with a Tech II so that 60MPH on the speedo is the true speed, this option will achieve the same effect albeit in somewhat of a workaround manner.

 

i am a new member and recently purchased a 2001 c5 convertible.it has 19" wheels on the front 20" on the rear. Obviously it has low profile tires on it. My concern is the accuracy of the speedometer. Is my year programable in the pcm and do i need tire size or tire circumfrence