3.55 and 3.58 ratio difference

Not sure if its a double overdrive but it does 100kph @ 1575rpm. How do you tell a double O/D? What rpm does yours do @ 100 kph? should be around 1475 rpm if your running 11r 22.5's.

 

The difference is about 11 RPM or .5 MPH. Those are basically identical ratios from two different manufacturers.

 

If you can do 58 MPH on a hill, you're not on a hill. You're going over a bump in the road.
600 HP notwithstanding.

 

So Minnesota has no hills around the river? Im not talking about mountains.

 

Roughly 1425 RPM with 3.90 rears in double over. You can call eaton with the serial number of the transmission and they should be able to tell you, but by simple math you can tell its a single over. Single over is .86 on the final ratio vs .73 on a double over. You've got the stronger transmission but unfortunately it being a single over is causing you some serious MPG. It would be a better bet to spend the money on a tranny swap versus the rear swap, and use the money you save in fuel to get the rears down the road and be a happy camper

 

All I'm saying is that 58 MPH @ 1300 RPM sounds like top gear for most trucks, and if you can do that, you're not pulling a hill. It's just a slight rise in elevation.

Flatlanders....sheesh.

 

A 20918b is a double overdrive. In Australia 4.11 and 4.33 are the most common rear ratios for American driveline highway trucks. Most trucks for b-double work will have 4.11 for around 140,000lb Anything over that will have 4.33 or even 4.5. With these weights high ratios and low rpms don't work and fuel economy suffers