cat c-15 18 speed 3.08 ratio???

Mine with 3.55s is 1450 at 70 but I have 11R24.5 drives. I don't know if they would work with your reefer or not but it could be a cheaper alternative at new tire time to go with taller rubber.

 

What's the final drive ratio of your transmission? With a .74 O/D and 3.58s, you should be running about 68 at 1500 RPMs with 3.58s and 11R22.5s. What exactly are you running, tire-wise? 11R24.5s rotate 476 times per mile.... 11R22.5s rotate 498 times per mile... 275/80R24.5s rotate 501 times per mile... 275/80R22.5s rotate 516 times per mile.. a 255/70R22.5 rotates 564 times per mile. The more tire rotations per mile, the more rotations of the rear end gears to travel that mile, which means the more rotations of the driveshaft, which means the more rotations of the transmission output shaft, which means the more rotations of the transmission gears, which means the more rotations of the transmission input shaft, which leads up to more rotations of the engine crankshaft to make that mile.

If you feel like tinkering around with some math, you can figure out what puts you where by...

(engine RPMs x 60) /(rear end gear ratio x tire revolutions per mile x transmission final drive ratio) = your speed in MPH at the specified RPMs with the specified drivetrain

So, let's say you had a .74 O/D transmission with the 3.08 rears and 11R22.5s....

(1450 x 60) /(3.08 x 498 x .74)

1450 RPMs would actually put you between 76 and 77 MPH (of course, your transmission might have a lower final drive ratio... I don't know what you've got)

 

So it sounds like you have 275/80R22.5s... with those, 3.08s would put you around 67 MPH at 1300 RPMs... 3.25s would put you at around 64. I'd assume the tires to be less expensive than the rear end swap, but that's initially, and I don't know if it'll save you money over time.... I guess it all really comes down to how much longer you'll be keeping that truck.

 

You're pretty much losing 1 mpg by pushing harder through the air at the higher speed.

I'd think changing ring and pinion, yes you'll still be at your desired engine rpm but the extra power required to force truck faster through the air will have your intake boost up and the MPG won't be as good as it is now at the slower speed with desired rpm.

Might be good though to be able to drop into direct and run 59 mph for some good mpg numbers with that gear change.

That's physics.

 

I would have to agree with semi crazy. Your rpm would be lower for the higher speed if you changed rear end ration but it would really cut your fuel economy which kinda puts you right back to where you are at.
There is different things you can do to improve fuel economy but higher speeds will kill the fuel economy especially over 62 mph it really drops economy even if your rpm's is lower.
If you want to drop some money to gain economy, think about swaping one of the drive axles out with a lift axle. That would gain you approx. 1 mpg.

 

High flow mufflers are a cheap way to add a little mpg. Running over 65 starts to make the mileage drop off. Also, running too tall of a gear will "lug" the engine and you'll get worse mileage.

 

The only way to improve your mileage is to start "fixing" that engine. There are many small, in dollars and work, upgrades you can do to get much better economy. Look in the http://www.thetruckersreport.com/tr...truck-performance-tuning-modifications-forum/ and read up to figure what is up, and what you can do to get out of the emissions hole.