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Large tires vs smaller tires

I have two dodge durangos. on one I have 235 75r 15 and get only 18 mph at best; and the other, I have 31x10.5x15 and get 20 mph at best. each came with the tires described so if I put 31x10.5x15 on the other one, will it get 20 mph as well?

Do they have the same engine, transmissions and axle ratios? Will you correct the speedometer/odometer for the tire size change?

If the two trucks have the exact same specifications that should work. Why not exchange tires and find out BEfore buying another set?

Probably not. But you could swap the wheels and do the math. Don’t forget to correct for odometer error. Drive at least 10 miles on the Interstate, based on the mile markers (not perfect, but pretty good) and see what the odometer reads for that distance. Adjust your readings accordingly, run a few tanks of gas through each truck.

And let us know. My Dakota has 31x10.5x15 tires, and I’ve been contemplating switching to 235-75R-15s.

20 MPH at best you say? Try to stay off the highway. What is happening is that the 31’s are a bit larger than the 28-29 inch 235’s. The speedometer and odometer will be off slightly, it will either read a little slow with 31’s or a little high with the 235’s. It depends on what your vehicle came with. It will also slightly effect the rear end ratio.

Actually you should get BETTER gas mileage with the 235. They are a skinnier tire…thus lower rolling resistance.