Why are my RPMs at 60 mph lower than they should be?
 

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?

How worn are the tires? How accurate is tach and speedo?

The tires are at 95%. I just got them in January this year. The tach seems pretty accurate, but I can't say the same about the speedometer. That's why I used my GPS to determine the exact road speed.

tachs normally read 50 rpms off or so. Do you have a qualcomm? They have a digital readout of rpms.

Hmmm....sounds pretty close to me

I have a C15, with 18 speed, 373 rears & that's about where I run at 60 mph, right around 1350, but then I'm running 11R24.5 tires, so by rights you should be a bit higher with low pro tires I think.

Is that correct guys...smaller tires makes rpm bit higher?

You trust the tach exactly, but the speedometer not at all. I sense a failure of logic here. :wink:

Is it a digital tach, or a random dial guage?


Yes Kat, smaller tires means you have to turn them more times to reach a mile. So for the same speed the engine needs to turn faster.

I have a satellite but it's not qualcom. I don't see rpms. I do have a GPS and it gives a pretty accurate road speed reading ...

Just a regular tach ...