I am shocked that I had to order a 3.92 gear in my new Ram. The dealer said given the 20”s, Dodge only uses the 3.92. I really wanted the 3.55 but it’s not available. With the torque and horsepower of the Hemi I can’t understand why they don’t offer the 3.55 or 3.21 with the 20”s. What gives? All the ratios are so close I can’t imagine any big performance loss and a 3:55 with 20”s would have pulled 244 RPM off the tach. at 75 MPH.

A Ram 5-speed auto. going 75 mph with 32.5-inch tires, 3.92 axle gears in Fifth gear (..67:1 overdrive) and high range on the T-case (1:1) would look like this:

20” tires

(75 x 3.92 x .67 x 1 x 336) = 66185.28/32.5 = 2068.29 RPM

(75 x 3.51 x .67 x 1 x 336) = 59262.84/32.5 = 1823.47 RPM

(75 x 3.21 x .67 x 1 x 336) = 54197.64/32.5 = 1667.61 RPM

(65 x 3.92 x .67 x 1 x 336) = 57360.57/32.5 = 1764.94 RPM

17” Tires

(75 x 3.92 x .67 x 1 x 336) = 66185.28/31.6 = 2094.47 RPM

(75 x 3.51 x .67 x 1 x 336) = 59262.84/31.6 = 1875.40 RPM

(75 x 3.21 x .67 x 1 x 336) = 54197.64/31.6 = 1715.11 RPM

(65 x 3.92 x .67 x 1 x 336) = 57360.57/31.6 = 1815.20 RPM

 

I don't understand it either...

Ours does bout 1500 RPM at 60 mph.

 

A gear change may be my first up grade. I have also heard rumor that manufactuers are looking at releasing more 20" tire sizes. A taller sidewall would help.

 

If I recall correctly it has more to do with rotational mass and rolling resistance of the increased size and weight of the bigger wheels.

 

you know I tought of that too, but I think the over-all dynamic mass of an aluminum 20 would be less than a chrome plated steel 17" wheel? Hopefully someone will be able to explain all this...guess I'm just stupid