What is the right rpm running at 58mph with 13 speed eaton fuller 258?

Answer 1

You're not looking at the full picture here. When you say "258", are you trying to make a reference to your gear ratio, and did you mean to say "3.58"? If so, it sounds like you're trying to run at 1325 - 1350 with a Cat engine, .73 or .74 OD transmission, and 275/70R 22.5s. Where you're at at which RPM depends on a number of factors... for one, the gear ratio of your transmission. Eaton Fuller does, or has, in the past, offered transmissions with a final drive ratio of 1:1 (direct drive), .87, .86, .74, and .73. So there's that to factor in. Then there's tire size and the number of revolutions per mile. Not all 22.5s or 24.5s are created equal. 11R24.5s will rotate about 476 revolutions per mile. 11R22.5s will rotate about 498 revolutions per mile. 275/80R24.5s will turn about 501 revolutions per mile, while 275/80R22.5s will turn about 516 revolutions per mile. 255/70R22.5s will turn about 564 revolutions per mile... so the "22.5" or 24.5" part isn't entirely what matters here.

There's a formula for figuring out where you should be at which RPMs...

(Engine RPMs x 60)/(Transmission final drive ratio x Tire revolutions per minute x Rear end gear ratio)

So, on the side, I do what's basically a daycab run with a 99 Kenworth T2000... this one runs 3.73 rears and an Eaton Fuller 10 speed with a .74 overdrive ratio, and 275/80R24.5s.

So, let's say I wanted to see where 1300 RPMs put me...

(1300 x 60)/(.74 x 501 x 3.73) = 56.4048

So I'd be running around 56-1/2 MPH at 1300 RPMs.

If this truck had 3.58s, I'd just swap out "3.73" for "3.58" and I'd be at about 58.7 MPH at 1300 RPMs.

You just adjust the numbers as applicable to your truck.