[DFS]

Have 289 (68 Fastback) with C4 Standard Auto Trans. My mechanic installed a 3:55 rear end. However I checked the actual speed with a GPS and at 30, 40, 50, the speed is 8 MPH slower than registered on the gauge. At 60, 70, & 80 it's 10 miles slower. Will for sure help me not to get a speeding ticket but how and what transmission gear with correct number of teeth should I try and get.?????
Thanks
DFS