Hey guys i just bought a 77 rally , i know you can tell the rear end gear ratio 2 ways, 1. on the front right axel tube its has alpha numeric symbols(TAC71) thats all i can read 2. inside the differential (GM 3984822 6:41 77) . What is the gear ratio from this information?

Are there any more numbers on the tube? There should be either 7 or 8 letter/numbers. I can't tell you the ratio from the info posted. The available ratios in V8 models with an 8.5" were 2.56, 2.73 and 3.08. For I6 with the 7.5" was 2.73.

My guess is that there is a 1 in front of that 6:41, which would make it 16:41, which is a 2.56 gear. Count the teeth on the ring gear and pinion gear. Divide the number on the ring gear by the number on the pinion gear, 41/16=2.56