tire height
 

I have an '04 crew-cab GMC 2500, with the 6 litre and 410 diff.
Will I be able to lower the rpm at a given cruise speed if I change from 425 to 625 tires? Right now engine RPM at 120 K/hr is 2700, and in road testing the 373 gears, the rpm's were about 700 lower at that speed.
I could change the diff gear, but warranty is on for another year, and I may be able to get some trade for the used tires.
Any thoughts. Thanks
ps My first post. Please excuse any errors and omissions.

Welcome to the forum.

Maybe it's different where you are, but where I live used tires sell for almost nothing. Used gears, especially 4.10, can probably sell pretty well.

Going to a bigger tire on a truck probably reduces MPG from aerodynamic drag and the extra weight of the tires. I am on a truck forum where lots of people use larger tires and lift their trucks and they always pay in FE.

Perhaps it would be a worthwhile experiment; try buying some used tires and see. Be sure to remember that your tire size is different, so your speedometer and odometer (and if you have a FE gauge) will read incorrectly. Either get that adjusted or use a GPS for your speed and distance when calculating MPG.

You might want to experiment with driving strategies first, to see how well you can do without investing in big tires.

Wouldn't bigger tires only pay off if the fe gained was greater than the fe lost by having additional weight? And if so, how would one go about measuring this? Some tires are rather expensive to be making a mistake...

What is the difference in tire height/rolling radius?

Yes, you will lower engine rpm at a given speed if you go to a taller tire, but keep in mind that the speedometer reading will then be off. Basically with a taller tire you will be going faster than your speedo indicates..

A GPS unit is a good way of finding out if your speedo is miscalibrated (mine is off by 10% thanks to shorter than stock tires)..

I would block the massive grill those trucks have in them. If you look behind it about 30% of the grill is blocked by sheetmetal before it gets to the radiator. So it shouldn't cause overheating. If it has a tranny cooler or oil cooler they are usually located behind the bumper slots anyway. Just a thought.

in my experience with my s10, going from smaller than stock tires to stock size tires paid off big time. granted it isn't 12' tall and 8' wide from the factory but the increase in distance traveled per rpm greatly outweighs the increased drag from vehicle height. that was a 205/60r14 to 205/70r14 (or actual stock 195/75r14)

3 things... 1. off-road lifted trucks usually have body lifts, suspension lifts which increase height but more importantly frontal area (suspension exposed to airstraem more). 2. they usually do engine mods 3. they usually don't get smooth tires, they usually get the knobbiest tires they can afford and drive as fast as they can stand the vibrations