I was thinking about my MPG's on my new 08 screw 4x4 and decided to try and figure my true MPG since my tires are larger than the stock tire (not by much, 265/70/17 vs 255/70/17) this makes me go 1 mph faster @ 70. It's a little unscientific but it works for me. I reset the trip in my truck and on my GPS and it works out that every 5 miles I drive by the truck odo I actually travel 5.2 miles so at 400 miles on the odo I take 400 divided by 5 gives me 80 now multiply by 1056 ( 2 tenths of a mile) giving me 84480 then divide by 5280 ( 1 mile) giving me 16. Thats 16 additional miles I actually traveled past what the truck odo reads so I actually drove 416 miles divided by X amount of fuel for real MPG on my truck, what I've noticed so far is about an extra 1 mpg so for all who have 33's, 35's etc. you may not have dropped as much mpgs as you thought you just need some new math for your figures.
bigger tires do 2 things one at freeway speeds they help with MPG in the way of lower RPM and two they take a lot more to get them rolling...so if you drive a lot on the freeway (18 wheelers) then the bigger tires are better...but you are right that some new math is a good idea....or just get the speedo and odomiter calibrated for bigger tires then there is a lot less math...
bigger tires do 2 things one at freeway speeds they help with MPG in the way of lower RPM
I did not have a tach in my truck at the time so I do not know about the RPM's but when I ran oversized tires on my tuck I went from 14 to 12 MPG. I preclocked my route so I already knew my mileage. When I put the stock tires back on my truck I jumped back up to 14 MPG.
Some trucks are "under geared" from the factory. My F-150 had 3.55 gears with a 5.0 and E4OD. Oversize tires made mileage even worse. If it had 4.10's things would have been better. In theory, lower rpm is good, but an engine running way below peak torque rpm can be less efficient.
In theory larger tires/higher gearing is beneficial as long as you have the power to turn them a lower RPM's..Diesels and big blocks would benefit most from this, although the $600-1000 you will spend for the tires will take you years to get back if ever. But the look is cool too and worth every penny if you need rubber anyways.