I have been rebuilding a 2001 Z28 SS SLP Camaro with an LS1 AND 4l60e Automatic transmission. This car had 17" tires on the front and 18" rims and tires on the back. I put the car back on the original 17" rims and tires front and back.


This past weekend I took it on a trip to check things out. I noticed the speedometer is way off (probably due to the tire sizes) but I don't know if they speedo was okay before on the 18" as the car was not running when acquired. Now that it is running, I find the speedo reads 90Mph for 75MPH actual as timed (i.e.-45sec per mile for 1 measured mile). Also, at 65MPH actual, the speedo reads 70 MPH. Obviously things need to be recalibrated. From what I can find out, this is no longer a matter of changing a transmission readout gear as in previous years. I would appreciate commentary on how to accomplish recalibration and reduce this substantial error.
Thanks,
Ted von Kampen
Nebraska

 

check with your local chevy dealer and see if they can set it for you. not sure but a tuner may be able to do it for you. check with solomon. LT1 PCM Tuning - Welcome!

 

Fyi, wheel diameter has nothing to do with speedometer calibration, overall tire diameter does. You can have the same height tires on different diameter wheels, and the speedo reading won't change.
Could be someone along the line changed the gears in the rear.
Depending on how much it costs to pay someone to tune your PCM, you might be able to find a used Hypertech Power Programmer for the same or less. With that you can recalibrate the speedo simply yourself, and other things as well.