There were three different gear ratios used for the MA70's 4.30:1 3.91:1 and 3.73:1. All three were available in LSD, the latter two were ONLY available in LSD and the 4.30:1 was ONLY available in the non turbo. If you spin one wheel with the back end jacked up and the other side moves in the OPPOSITE direction it's an open LSD and thus it's 4.30:1. If both move the same way it's an LSD. If it's an LSD you may look at the charts below to easily find which one it probably is. The minor difference in gearing will make for a small offset.. so will the MASSIVE margin of error in the stock spedometer and tachometer. Best way to check is to roll the car along a measured lengthof road and check against the odometer.
75's right for 5th gear in a N/A at about 3300RPM.
W58: (that's the non-turbo transmission)
Attachment 3095
R154's:
Pre89 (3.91:1)
Attachment 3096
89+ (3.73:1)
Attachment 3097