If I change from 265 to 285, will mileage go up or down -- increase or decrease? Less tire revolutions per mile, so less engine RPM at a given speed. But, more rolling resistance. So, will mileage go up or down?
JW,
If you switch your mileage will APPEAR to decrease, but it may not actually decrease.
The 285's will turn fewer revolutions in a mile than your old 265's, so your odometer will register something less than a mile with every true mile you drive. This will appear to you as a decrease in fuel mileage.
The only way to be sure is to calibrate your odometer (both before and after the switch). Doing that is not as easy as it sounds. For example, you can't use the mileage markers on freeways. Surveys are done in horizontal miles, but you drive up and down all those hills, so you will drive farther than the measured mile. Also, are those mileage markers for the center of the freeway or what? One thing's for sure: They're not for the lane you're in.
I like to calibrate my speedometer (rather than the odometer). I use the temporary radar signs that the cops put up -- the ones that say, "Speed limit 45. Your speed is 62. " It's usually hard to find one of those signs on a low-traffic road, but when I do, I check my speedometer.
I've been told, but haven't been able to confirm, that if the speedometer is correct the odometer will be also. The new trucks use electronic instruments, so this sound right to me. If that statement is true, then if the odometer is right, the speedometer will be too.
The other way to calibrate your speedometer (or maybe it's the odometer) is with a GPS. I don't have one, so I haven't used this method. I like the idea, though. Those things are really accurate.
That's the long answer as to why your mileage will appear to decrease, but may in fact increase (due to higher gearing).
By the way, where did you get the idea that larger tires have more rolling resistance? I haven't heard that before and am not sure it's true.
Good luck with whichever decision you make.
Loren