For those that have the 3.92 gear with the factory 20” or 17” rims/tires I have a question. When the truck is fully warmed up and you’re traveling at an indicated 65 mph on a level road WHAT IS THE RPM OF THE TACHOMETER READING? Please list if you have the 17” or 20” rims. I need to know both. If you have 3.55’s…..please refrain from replying.

Thanks

 

Its the overall tire size and rear gearing thats important not wheel size.

 

I was driving yesterday at 60mph. Indicated rpm was about 15-1600. So at 65 you should be seeing about 1650-1700 or so. My truck has the factory 20's and 3.92 gears.

 

3.92 w/ 20" stock = 1700 @ 65mph

 

Abolutely right! Wheel size play no part in this only the tire size.

 

Originally Posted by 08DodgeTRX View Post
Its the overall tire size and rear gearing thats important not wheel size.

If you both go back and read my post I stated that I'm interested in the "factory stock wheels and tires" when making this test. I know what the factory tire size is for the 20" and 17" 4th Gen Ram.

The 20" has a diameter of 33" and the 17" has a diameter of 31.7"

 

the factory 265-70R17 tires are 31.6" tall and the 265-60R20 are 33" tall. With the 31.6" tires, the RPMs are, 3rd gear=2706 RPM, 1st OD=2030 RPM and 2nd OD=1813 RPM. For the 33" tires the RPMs are, 2595, 1947 and 1739 respectively.

 

Good thinking Pedro Dog. I found a rpm calculator on the web that confirms what you posted.

THANKS!

 

There are 2 different size stock 17" tires. The TRX uses LT275/70R17

 

Ouch! Just tryin to help you bud, to my defense it was confusing. Anyhow here's what you need. Ill even through in the OGR.
Find overall gear ratio- (3.92 gears is 2.6264)
OVER DRIVE GEAR x REAR GEAR = OVERALL GEAR RATIO (OGR)

FIND YOUR RPMS at ANY SPEED-
SPEED x OGR x 336 / TIRE HIGHT = RPMs

You can figure it out yourself now