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I have a new 09 sport with 3.55 rear and 20in wheels. Dad has 04 3/4 ton with hemi and same trans but with 3.73 rear and 17in wheels. My question is why are we running the exact same rpms while driving. I thought I would be less rpms than him. I could see the same rpms if I had the 3.92 rear but I don't. Just wondering.

Eslund
 

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I agree overall you should be running lower rpms at the same speed. The 20 inch tires i believe are a little taller than the 17 inch so theoretically you have a taller gear in the rear and a taller tire which is equivalant to a taller gear also. I have heard that the 3.55 and 17 inch wheels are the equivalant to 3.92 and 20 in overall ratio. I could be wrong but that is what i think.
 

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your hemi is bigger than his hemi
 

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Your overall tire diameter is basically 33" provided you have the stock 275/60/R20's on your truck. Your rpm at 65 mph should be approximately just under 1600 rpm(1574 actually). He needs somehere between a 34.5 and 35" tall tire on his truck to equal your rpm at highway speed. If his truck has that on it there is your reasoning. Error in reading the tach comes into play here too. Even with a 33" tire his rpm is still only 1650 rpm at 65 mph anyhow. This is all calculated with a .67:1 overdrive ratio and assumption of no slippage. That's correct for the 545RFE and when cruising the torque convertor is locked so slip is near zero..........
 

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09 has 275-60-20's and the 04 has 265-70-17's. Just bugs me that they run so close in rpm's. Still don't think they should.

Eslund
Pluggin the numbers into a tire size calculator, the 20" tires should be about 33" tall turning 611 revs per mile. The 17" tires should be about 31.6" tall turning about 638 revs per mile. Rpms should be higher on the 17 tires. Add in the fact that he has even steeper gears, that should increase the difference even further. So if your numbers on tire size are correct, there is an issue with the gearing be it the tranny or rear end as they should not read the same Rpms. Be sure when you are checking that you are in the highest gear and locked up.

Did you get limited slip? If so, you would have the 3:92 rear end. Definitely something I would want if you have any hilly terrain to travel though.
 
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