Originally Posted by
Chris@HPTuners
the problem here is GM. You see, in the code that calculates the vehicle speed, the PPRev value isn't actually used and there isn't actually a tire size value in the calibration! PPRev is only used to calulate the trans output RPM for automatic trans vehicles and is almost always calibrated incorrectly for manual trans vehicles.
So what you find is that "almost all" manual trans vehicles are fitted with a 17 tooth VSS sensor but the PPRev calibration incorrectly shows 40 teeth (the usual VSS sensor for a 4L60e). To avoid this problem with almost every manual trans LS1 vehicle we have to assume the VSS Sensor is 17 teeth(even if the calibration shows it as 40).
Now, for some manual trucks they actually are fitted with a 40 tooth sensor (like a 4L60e) and our calculation breaks. There are also about 5 other stupid ways GM calibrates the speedo for LS1 trucks.
For those vehicles you'll need to scale the PPM manually.
This is one area GM is horrible at calculating, even the latest code implementation is very poor.