I had a 93 5 speed cummins that comparing it to. The 93 was the strongest truck I have ever pulled with but had 4.10 rear end and would only run 82 WOT. The 91 doesnt even like 5 gear til around 60mph...The 91 was a compant truck and had too fast engraved on the ignition key..
Easier than pulling cover is to jack the rear up and put it in neutral. Put a mark on the driveshaft and on another on the tire and spin the tire one full turn while counting shaft revolutions. Shaft spins 3.5ish times, it's 3.54, shaft spins just over 4 times, it's 4.10. Of course you still have the same gear oil in there, as opposed to the pull-cover procedure.
Im almost positive i have 3.54 if so that will change. I havent had this truck long enough to do the complete running gear check that I do before driving one much...It will have 4.10 or lower soon. I am too old for speeding just want to pull the fifth wheel across Monteagle mountain on I24 at a decent clip.
Here is how you tell if you have the 3.07 ratio axle informally called the 'Dana 71'. The housing is cast differently to fit those tall gears. The pronounced corners circled in green are even more evident when the diff cover is still on.
3.07 gears were still an option up to 93. There are a few people on the forum with factory 3.07's in the intercooled trucks with the getrag or 518. I know Phillip has factory 3.07 and his is a 93 I believe. He will probably chime in
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