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I created a tire size/gear ratio calculator spreadsheet a few years ago and just modified it for the new 66/68RFE ratios. I am considering ordering a new 2500 Hemi with the new 66RFE and wanted to compare my gears with the 545RFE with the new trans ratios. When comparing my truck with 3.92 gears and 33" tires and a 66RFE truck with 4.10 gears and stock 31.6" tires, the 66RFE is at almost the same RPM in 6th gear as the 545RFE in 5th gear. Then comparing 4th on the 545 to 5th on 66, you actually get a sensible RPM increase, not just a 200 RPM bump. Then if you change the tire size on the 66RFE to 33", the final drive RPMs are actually less on 66RFE with 4.10 gears compared to my current 3.92s...

I think since I plan on going with a 33-35" tire, I will go with 4.10 gears... I like the fact that I can cruise at 75 mph and still be under 2000 RPM...

Feel free to manipulate the spread sheet to fit your needs. All of the formulas are locked so you can't mess them up. Also, only rows 3 and 4 are editable on the gear ratios and tire size. Everything else self populates.
 

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Nice write up.
 

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Very NICE!!!!! Looks like 4.10's are a great choice for a HEMI Powered 2500!
 

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Thanks so much for this info!
 
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