I have an 88 S-15 Jimmy I'm building for the kid. We're going to do 35's for tires and I know I will need to re-ratio the front and rear diffs. Is 4.11 the lowest ratio I can go on the front or are there lower ratios out there now? With 4.11, will the speedo be close or will I ve to do more to get the speedo to read correctly, the truck has 3.54 stock?
Thanks guys!

 

They make 4.56s...but I doubt you'd want to go that low. Also you can just reflash the computer via dealer or programmer instead of trying to match tire size + gear ratio = stock speedo range.

Sounds like its a pretty sick jimmy by the way, you should post some pics!!

 

I agree, I wouldn't go any lower than 4.11.

For an '88, I'm not sure if the speedo correction is something that is dealer programmable, if it is as simple as a gear in the transfer case, or a chip change...

With 3.54:1 ratio (assuming auto locked in third gear, the torque converter fully locked, and that you have stock 29" tall tires), with 29" tall tires (695 revs/mile), you will be at 2460RPM at 60MPH. With 35" tall tires (576 revs/mile) and the same 3.54 gears, you will be at 2040RPM at 60MPH. To get back to the same RPM at 60, you will need 4.27:1. 4.11:1 gears would give you ~2370RPM which isn't that different. So I would stick with the 4.11s.

 

Thanks guys! Not really anthing worth posting yet, still doing bodywork, but I'll post some when it's done.