Obd2 speed doesn't match
 

The guy tuning my car ran into an interesting situation today. He said when checking the speed through the OBD2 port it showed twice what it should have. The speedometer is showing the correct speed and the transmission is shifting correctly.

The car is a 1984 Corvette with a 2001 Corvette LS6 motor and a 1999 Camaro 4l60e transmission

Any ideas what parameters he should check?

https://cimg1.ibsrv.net/gimg/ls1tech...65fc37d12.jpeg

Does this look right?

What is your speedo driven from...?

The speedometer is driven from the speedometer output of the ECM and goes through a Dakota digital box to calibrate the 1984 Corvette Digital speedo.


The speedo and the transmission shifting are accurate, it's the obd2 output that looks exactly double of what's on speedo.

The Dakota box can be configured to halve or double the pulses per mile, which may have been necessary to get your speedo to work correctly.
That could explain the difference.

On engine swap vehicles I always set up the speedometer in the pcm and get it reading correctly (correct tire size and gear ratio) and verify it with gps. Once the speedometer is correct in the computer then I worry about the gauge in the car since 99% of them have to be programmed also.

Is it simply a matter of setting the correct tire size and gear ratio for your vehicle in the PCM, then changing the number of pulses per mile that it outputs to suit the speedometer? I'm currently about to fit a Stack dash from a S1 Lotus Elise into a LS1 Miata, and would like to avoid having to use an external converter if it can all be done in the PCM.

The Stack speedometer is expecting to see about 35,333 pulse per mile, based on the OEM tire size of 225/45-16 with a 42 tooth wheel.