My friend just recently bought a 95 TA M6 conv. She has noticed that her speedo has been out by 10 miles, when she is doing 80 she is actually doing 70mph. The car has 255 Falken tires on the stock 16 inch rims, will a larger tire like this put her speedo out by 10mph? I was thinking that the person who owned it before her might have installed gears? What are your opinions? Thanks for the help. Pontiac said that they did not know what was wrong and they were not able to program her PCM to fix this error. Where could she send it?

 

Assuming the car came stock from the factory with the 245/50's, running a 255/50 will cause the speedo to read about 1.5% low. That means at a true 70mph, the speedo would be reading about 69mph, not 80.

If its reading 80mph, when it is actually going 70mph, that's about a 15% error. That could only be caused by a gear change.

But the M6's came with 3.42's stock, and 3.73's would only cause a 9% error, and 4.10's would cause a 20% error.

Is it possible someone put a 155mph "overlay" on a 110mph speedo?

 

thanks for the help Injuneer, I think that I am going to drive side by side with her and match up our rpms and see if it could be a gear change, it is weird why it is like this. Thanks for the help.