Hey dsy1, I'd like to know what's up with this one? Please do this little test some day. Test it in the morning then take a short trip the other way and see if it's directional or if it's the temperature related. Do the same in the afternoon. I use a gps to set the spedo's on Kenworth truck's all the time. Nothing like a trucker that think's there not going fast enough or loosing miles. C-ya Scott