gear choices help calculate please

BIG JUICE · Nov 19, 2018

hello guys, I have factory 3.55 gears with 37" toyo rts. I felt undergeared with stock tires but now it really lugs at about 1000-1050 rpms when it runs all the way into 6th gear. Yes, I know to lock it out, fix my shift points or use tow haul. But 95% of the time its in too high of a gear and lugs. I blow smoke and then it spools. I feel like a regear is the best possible choice to fix fuel economy, lugging and downshifting.

Do I use 3.73 or 4.11 with these size tires? I do not pull anything heavy currently and I spend lots of time on the highway at over 70 mph. I think 4.11 are what I need but I still like running 90 without screaming. any imput is appreciated

JHTX · Nov 20, 2018

Calculation is pretty simple it’s just a ratio of tire size to gear. In my case it was 32/3.31 = 37/X. Your left with x = (37 x 3.31)/32, which would give you 3.8 new gear ratio. The nearest gear would be 3.73, but you could go with 4.1 if you were hauling or wanted more power in your lower end. I also checked the RPM charts and this puts me about back to stock curve. If you want a factory option the only thing available is a 3.73. If you got aftermarket Nitro or Yukon you can get the 4.1, but you will need a slim gear ring for the front Dana, so you don’t have to replace the carrier. Note I had 18” wheels and 3.31 gearing. If you had 20” & 3.55 it still comes out to x = (37 x 3.55)/34 so 3.86. I think personally it comes down to preference on how much power you want in the lower end to get the tires moving and how much towing you do. I went with 3.73, as I float between 37s and 35s as I have a Carli 2.5” lift, and did now want to be over geared if I go back to 35s. I have never noticed my truck struggling with this setup, but I sometimes wonder what a 4.1 would drive like. I think larger gears is a by product of the older 4 speed transmissions, with the six speeds you may not need as high gearing in differentials as the transmission gears are closer. Hope this helps.

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BIG JUICE · Nov 25, 2018

That was an awesome response. Thanks. I was I guess looking for actual rpm change after final drive and transmission. I am unsure how to specifically calculate the rpm

8WR_ZJ · Nov 25, 2018

http://www.dieselhub.com/tech/rpm-calculator.html

I have used similar calculators to do the same.

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JHTX · Nov 25, 2018

Appreciate it. I researched the hell out of this topic. I actually found a website that shows the RPM to speed tables for various gearings with 37” tires. Think it was a crawler page.

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