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What gear ratio running 65mph @1350rpm with and 18 speed

I just bought a 2014 389 peterbilt with a isx 550 Cummings, 18 speed RTLO-20918B with DANA-SPICER D40-170 40,000 lbs full locks and 3.91 ratio on 22.5 tire. My question is. What gear ratio should I have to run 65mph @1350rpm in 16 gear or 17gear. Thank you

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In your transmission 16 gear would be direct drive, 17 gear would be your first overdrive, and 18 gear would be your second overdrive. With your set up, I would run 2:64 gears and mostly run in 16 gear. If you need to go faster you will then have 17th gear. 18th gear is just to fast to be a useful gear. Your Rpm may be closer to 1450 at 65mph in 16 gear. If you want it down to 1350 maybe see if there is a gear ratio available in the 2:50s. But I think a 2:64 would be a huge improvement over what you have and make your truck much more drivable. 3:91 is geared way to low for 65mph with that engine.

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Go here and you can input what you know. It will calculate for you:

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