[Big Dave]

A few years back I experimented with LT225-75x16E tires instead of the LT235-85x16Es the truck came with. That resulted in a slight loss of MPG.

But that was before I got my GV on and sorted out. At 70 MPH I was revving 1,685 RPM with 235s and 1,825 RPM with the 225s.

Now with the GV (and its 0.8:1 drive ratio) I turn 1,325 RPM @ 70 MPH. With 225s I’d expect about 100 RPM higher. Still 35% slower engine speed at the same road speed than stock.

With the 235s I run EGT right at 600 degrees and can’t seem to coax it down much. Trials with 225s show temps 125 degrees cooler. Lower temp indicates lower fuel burn.

Lower temps are also good for extending engine service life.

If you look, all the cars that get outrageously good MPG have teeny little wheels – generally 14 inchers. That’s because they are designed to be urban cars. Lots of stop-and-go driving.

Your drive train doesn’t accelerate the vehicle away from a light. It accelerates the wheels/tires. The tires grip the road and that equal and opposite reaction thing accelerates the vehicle. You have to pour kinetic energy into the wheels before they move your vehicle.

The size of the “flywheel” is measured by the moment of inertia of that wheel. Before I stated the moment of inertia goes up with the square of the diameter.

My bad. Moment of inertia (according to my Marks handbook) goes up by the FOURTH POWER of the diameter. So 225-75s have a moment of inertia 30% less than that of 235-85s. Also the tires weigh a third less.

Thus I will expect the truck to require less fuel for stop-and-go driving (about a third of my driving) and the penalty for 100 additional RPM may be offset by cooler engine temps.

We’ll see.