Here's a question for you guys ... At 60 mph the engine is supposed to turn at the number equal to: Tires revs/mile X Top gear ratio X Rear axle ratio, right? For example, if your tires do 495 revs per mile, and your tranny has a 0.73 ratio in the top gear, and your rears are 3.90, then the engine should turn at 495 X 3.90 X 0.73 = 1409 rpm at 60 mph.
For some reason, this straightforward formula doesn't work with my truck
(2004 International 9400 with Cat C15).
I crawled under the truck, cleaned the grease from the front diff - the plate says, "373" ... so my rears are 3.73. I called International Truck Call Center, gave them my VIN and they confirmed - the top gear ratio in my 13-speed is in fact 0.73. The drive tires are Yokohama TY517 low profile 295/75R225 and according to the tire maker's website they do 512 revs per mile. So, at 60 mph I should get 512 X 3.73 X 0.73 = 1394 rpm.
Here's the real life numbers: 60 mph - 1325 rpm; 58 mph - 1300 rpm.
I checked and doublechecked everything (tires, rears, tranny) and the RPM is supposed to be higher. Somebody else would be happy to be in my position since they could drive faster and save on fuel but for me the purpose was to slow down, and I can't efficiently use the truck at 57-58 mph because the rpm are too low and the truck doesn't pull well on hills especially when fully loaded.
Any ideas? Why the discrepancy?