Differences b/w available axles

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Apr 25, 2016 | 08:33 AM

#31

Rasalas

Cargo Master

Joined: May 2007

Posts: 3,336

Likes: 4

From: Western New York

Quote:

Originally Posted by cyncwby

Rasalas ..... no problem on contradicting me but me thinks something is amiss. The rpm calculations I got off the enet are correct, the Michelin Defender 275-65-20 are in fact 34.1" in diameter. It doesn't matter what the aspect ratio is the calc's go by diameter of the tire.

Not a math major but I find it kind of facinating that the rpm difference between a 3.73 and 4.30 with the same diameter tire gets higher as the speed increases. At 60 mph the rpm diff. is 226, at 65 = 244, at 70 = 263, at 75 = 283 and at 80 it's 301.

Somewhere I had the wrong diameter on tires or maybe not the correct information on OEM tires. As for the difference a simple way to see the increased difference is to make a simple graph and plot the individual ratios. They all start a 0,0 but diverge as the rpm's increase.

Back in the days when a New Process Gear 4 speed was considered a good transmission the final ratio was 1:1 in 4th gear. I had a 3.73 rear end pushed by a 400 Windsor engine. My friend had a 4.55 rear end with the rest of the driveline just the same. Our driving was similar. I got 11-12 mpg, loaded or empty, fast or slow. He got 7-9 mpg with similar habits. Ever since that day I have been concerned about over revving engines and the effect on fuel mileage. Granted, the trucks today are much better engineered to handle loads and variations in engine speed but a 10% difference in fuel economy adds up quickly and a 25% difference is astronomical.