Join Date: Sep 2014
Location: Summers in YT / Otherwise SWPA
Thanked 83 Times in 75 Posts
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Yeah - personally I like the 3.42 gears for interstate driving and just dropping into 5th when the load demands. With the lower 1st gear of the aisin, I think it works fine, but then I'm not approaching the upper weight limits. I do think, unless yours is a grocery getter, that the tire size increase will leave you feeling overgeared. Anyhow, just take the two tire diameters, multiply them by Pi 3.1415 is close enough, then divide which ever one you're considering going to by the one you have (works for both going up or down in tire size. The answer will be the percentage change in your RPM, so just take your RPM now at the highway speed you like and multiply it by the percentage you just calculated. That will give you the RPM you'll have with the new tires. I suspect with the 68RFE, if you're empty and cruising 80 MPH interstate you'll be happy, but if you work that truck at all, you'll not like the higher gearing.
Finally - I got to put that ol high school math I learnt long ago to use.
2015 RAM 3500 CC LB SRW, 4x4, Aisin, BigHorn, 8.4an, Husky-X, Duraflap, cabin air filter, LTX MS/2, Maxiscan MS310, Super Z cables, US Cargo 60K recovery strap. Also 1998.5 NV4500, RWD