K thanks I will try that after I get paid this coming weekend to get another obd2 reader as my old obd2 bluetooth reader has crapped out.Ok it's an error of percentage. That's why the amount of error is greater at higher speeds. 10% at 20= 22 or 18. 10% at 60= 66 or 54 . Clear as glass? So one way is blast down highway, call up smartphone GPS , or another GPS device. Drive steady speed, record indicated from tach and speedo, and what does ecm say. Against GPS speed. Result is how much and where errors are. That's what I did. Little larger tires 66 indicated is actually 70 ground speed checked by GPS.