Speedometer Calibration - where?

[BlackMetal]

Going to be running 28" tall tires this summer and want my ECM to be getting an accurate vehicle speed. I don't really think it's something I feel like screwing with myself. Anyone know where I can take it?

 

[Roger]

Speedo Caliration


I use the interstate mile markers & a stopwatch.
The mile markers are within inches of a mile apart.
DOT require accuracy. You can verify accuracy by using many different mile markers.

Set cruise control at 60mph & time between markers should be 60 seconds. If not 60 seconds, either mathmatically calculate the difference, or adjust cruise control until you take 60 seconds between markers & you can adjust your driving style or change the speedomer pickup gear in the transmission.

 

[jcj53115]

When ever I have had to recal a speedo I have a local trans shop that does it for me kinda. I just call him and give him the needed info ie... trans, gears, tire size ect and he gets the correct speedo gear for me. Easy wnough to do your self really, just a small plastic gear in the end of the trans.

 

[WSLN 6]

It is the black gear you will be looking for at GM to replace the stock tranny gear.