Ive got a 91 7.3 NA with a zf 5spd, it originally had a 4.10 rear end, and since I don't tow heavy with it, i wanted to swap the rear end to gain some mpgs and speed on the highway. My dad had 3.55 put in it, and when he got it back, he did an rpm check. The numbers he reported to me were the exact same as with the 4.10. 2200rpm at 65 and 2500 rpm at 75. I didnt think that sounded right for the 3.55, so today he went back out and checked with gps. GPS speed at 65, speedo at 58, and 1800 RPM. GPS Speed at 75, speedo at 66 and RPM at 2100. Good those are the RPMs I wanted to see at those speeds.

(Funny that his initial check of 75mph at the speedo was actually close to 90 mph with the old girl, on a 65 mph highway!!).

So anyways, is there a way to hone in the speedometer so that the reading is accurate?

 

yeah ... keep the GPS in the truck .... thats what i did ... I didnt swap gears but i went from 33's to 35's with 3.55's ... big difference

need to change the speedo gear in the trans if its 2wd or in the tail of the T case if its 4x4 .... tranny shop can hook you up with the right one for the gears you have now

 

It is a 2wd. I read that getting at the gear in the transmission is a little tricky in the 5 speeds. Is it really that difficult

 

No ....... but depending on The set up ...... u may actually want to pull the crossmember out and lower the trans with a jack to access easier ..... but I do recao its do able with out lowering trans ..... I could be wrong though .... been a while sinse I workid with a 2wd ZF trans