I have a 72 Fj40 with 38.5 tires, 350 motor, SUA and stock transfer case. When I purchased the truck it had 35 inch tires on it with 4.11 gears. It work just fine like this but now it could use some gear. If i did the math right a 5.29 would get me lower than what i had before and shoud work good. Is there any problems I will have or run into on the install and making them live. The other question is do I need to install something to control axel wrap if it is SUA.
Gear Change Questions NEED HELP PLEASE
- Thread starter mlrfj
- Start date
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If you have stock semi-floating rear axle, you will have to grind the teeth of the 5.29 gears to get the center pin to fit. Besides you'll have a really small pinion. 4.88 is typically as low as most Cruiser owners go in the diffs. What tranny do you have? Stock 3 spd?
You should have no problems with axle wrap if you're SUA.
You should have no problems with axle wrap if you're SUA.
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That's up to you, where you want to take it and what you want to do with it.
Looking at this gear ratio calculator:
Gear Ratio Calculator
I plugged in your numbers and your low crawl ratio is about 35:1. Stock is about 28:1, but the auto makes up for extra gear reduction. 5.29s are also a little harder to find, so if you broke one (which could be possible because your getting into small pinion land at that point too) it's not likely you'd find a replacement easily, especially if you were on the middle of the Rubicon or something. 4.88 is a lot more common and easy to find. That's probably as low as I'd go in the diffs.
If you really want to go slow you can always put an Orion on. 3:1 or 4:1 reduction on your t-case.
Looking at this gear ratio calculator:
Gear Ratio Calculator
I plugged in your numbers and your low crawl ratio is about 35:1. Stock is about 28:1, but the auto makes up for extra gear reduction. 5.29s are also a little harder to find, so if you broke one (which could be possible because your getting into small pinion land at that point too) it's not likely you'd find a replacement easily, especially if you were on the middle of the Rubicon or something. 4.88 is a lot more common and easy to find. That's probably as low as I'd go in the diffs.
If you really want to go slow you can always put an Orion on. 3:1 or 4:1 reduction on your t-case.
Mark W
Yep, I really don't really care that much I guess.
The reason that most don't go lower than 4.88 is simply the amount of grinding that is usually seen on the ring gear of 5.29s to install in a semi-floating rear end. Pinion strength will not be an issue except in extremely rare instances. ANYTHING *can* break after all.
What many people fail to pay attention to is that the pinion shaft will align differently with the ring gear teeth every time you rotate it to the next bolt hole . It is possible to put it together so that the amount of grinding needed it crazy. or you can align it differently and the amount removed will be much more reasonable.
I'm running 5.29s (in an FF rear). My usual trail partner is running 4.88s. Both V8'd with 40x18 Boggers. I am very happy with my setup... but IMHO the 4.88s will be a better choice for you if the rig sees street use as well as trail.
Mark...
What many people fail to pay attention to is that the pinion shaft will align differently with the ring gear teeth every time you rotate it to the next bolt hole . It is possible to put it together so that the amount of grinding needed it crazy. or you can align it differently and the amount removed will be much more reasonable.
I'm running 5.29s (in an FF rear). My usual trail partner is running 4.88s. Both V8'd with 40x18 Boggers. I am very happy with my setup... but IMHO the 4.88s will be a better choice for you if the rig sees street use as well as trail.
Mark...
- Thread starter
- #7
Thank both you for your help. This has gave me a lot to think about and after looking at the calcator it looks like 4.88s will get me back close to what i was with 35s. It also sounds like a lot less work. The truck is going to be used as a all around truck ( Mudd, light hill climing and street).
