AccessBowtie

Finally got the new gears installed on my '94 6sp Z28.
AAM 3.73's from New Era Performance.
They are very quiet. So for those looking to upgrade, I recommend AAM's.
Solved the clunking noise coming from the back of the car at low speeds.
Dealer said there was a lot of play in the stock 3.42's teeth.

So now that I got the new gears installed, the Dealer said they got a code
from GM that recalibrates the PCM so the speedometer reads proper.
But to me the speedo seems to be running about 3-5 mph faster than what I am actually going. (75 mph feels like 70)
Dealer said that the speedo is sensor driven, not gear driven like the old days.
They said what they did is the best they can do.

Question is for those that have changed rear end gears.
How did you guys get the speedo calibrated and how accurate can I expect it to be? Did you use a Hypertech programmer? Or get a new PCM from
Madz28?

One thing I loved about the 3.42's is that the speedo was 100% accurate on my car. So I knew when the speedo said 70 mph, I could count on it to me 70 mph. I'd love to get it back to 100% accurate again.

(for info, I'm running stock sized tires)

Injuneer

The Vehicle Speed Sensor (VSS) is located on the output shaft housing of the tranny. You have to put the new gear ratio into the PCM program. Either a used Hypertech progammer (insure the previous owner returned his PCM to "stock" or the programmer will not work on your car), a mail order tune (MadZ28 or pcmforless), or using programming software like LT1_Edit are the ways to correct the PCM.

Not sure what "the Dealer said they got a code from GM that recalibrates the PCM so the speedometer reads proper." means. Did they already attempt to program your PCM? They don't need a "code"... they would need something like a Tech 2 bi-directional scanner to make the changes, or they could reflash the PCM. Are you saying they did that and your "seat of the pants" meter tells you the speedo is still incorrect? They should be able to enter the EXACT gear ratio, and the speedo should be as accurate as it was before.

First, use the mile markers on the Interstate and a stop watch to figure out if your speed is in fact 3-5 MPH off (the 3.73's would cause your speedo to read 76.3 MPH at a true 70 MPH with no correction). If it really is, find out what the dealer actually did. There is no reason he can't get it exact if he did it the correct way.

AccessBowtie

Sorry it's taken me so long to respond.
I spoke with the Dealer. He said they downloaded info from my car using a Tech 1 tool. They used the Tech 1 tool to give the GM computers info about my car. In turn, GM downloaded info back to the Tech 1 tool based on the fact that I now have 3.73 gears. The Dealers said they then load that new info from the Tech 1 tool into my car. He said GM says there is a 3-5 mph speedo error because that's the best they can do. I have no idea why they cannot get it exact. But whatever.

I've had my wife follow me in her 2004 Grand Prix. At 30 mph in the GP, my speedo reads 31 mph. At 60 mph in the GP my speedo read 64. So it's definitely off.

So what's the easiest/best choice on getting the speedo error corrected?
The Hypertech or a PCM tune from one of the above listed places?

Injuneer

Depends on how cheap you can find a used HPP. Compare it to the economy mail-order tunes from these two sources:

http://www.madz28.com/

http://www.pcmforless.com/

Or, get a cable from:

http://www.akmcables.com/

And get one of these to allow you to "tune" it yourself:

http://www.carputing.com/

http://www.tunercat.com/

The story about the Tech-II sounds like they fed you a line of pure BS.