On the other hand, there's reason to suppose that more energy is lost in sidewall flex in a higher aspect ratio tire than a lower aspect ratio tire. Ergo a 65% 15 inch wheel with the same width as a 70% 14 inch tire may have less rolling resistance.

Driving around town this question is to complicated due to accelleration, deccleration and tire construction when turning.

In the special case of long distance driving we can make a simple case

-most gas consumption in this case is mainly replacing energy lost to wind resistance and friction. The other factors are not that significant.

- rate of gas use is related to rpm and the power curve for the engine. Regardless of what gearing you have in between all you want to know is how many times the engine turns over per distance actually travelled (not what the odometer says).
That is why we have overdrive, drive in 5th gear instaed of first, and we know higher ratio rear ends give better performance and poorer mileage.
We are just manipulating the same numbers by changing tire radius.
By extrapolation you wouldn't use 7 inch wheels and expect efficiency(your RPM would be doulbed).
Also your gas engine is upside down compared to an electric engine it has it worst performance at 0 rpm and the electric motor which is most efficient at 0 rpm. (that is why I would rather have a gas engine connected to a generator and an electric motor at each wheel-not my idea Dr Porsche toyed with this in 1925, the 4 motors)

Generally at high speed cruising lower RPM will also put you in a more favorable position in the power curve of thwe engine

Don

I had a 1994 accord with 195/60r15 tires, speedo was quite a bit off, says I was doing 70, when I was doing like 66. I put 205/60r15 tires on it, and the speedo was much closer, 70 indicated was 69. I just bought a 2002 Civic Si, recommended tire size was the same, and it came with guess what? 205/60r15 tires on it. I checked the speedo with a GPS,and its really close. just about identical to my accord. Maybe it was just a fluke, but it seems at least according to the speedo, one size larger is what it should have on it. Well it also could be my GPS if off, but I doubt it...

I know on FWD mopars they have 3 available speedo drive gears, 19, 20 and 21 teeth and a dozen possible final drive combos and a dozen factory wheel/tire size combos at least... so it gets the speedo gear that's within 5%...

...and one case where taller tires can almost certainly increase FE: My car's highest gear is way too low for highway use. I get into 5th at 30 mph, sometimes lower. The same gear runs at 3000 rpm at 70 mph. It's pretty lively on the highway without downshifting, but I wouldn't mind downshifting to get some acceleration. The car needs taller gearing (ideally through wider ratios, but that's not feasible), and the cheapest way to get it would be taller tires, though it wouldn't be a huge difference.

Depends on your driving habits, gearing, ect. A taller tire will give you different overall gearing and can help mpg...just don't go too big and go the other way.

I just changed my front tires from 195/70/R14 to 185/75/R14. It feels like it rolls better, but no numbers as they were changed mid tank. I got new steel rims with the front tires,so I may replace the back tire soon and just store the wider tires for short term need. They are only $35 each.

Slurp,

So what was the result of going with the taller tires? The difference wasn't much, but in terms of mpg, was there an increase or not?

I did that before I measured FE. sorry :(

You gotta consider the effect the larger size has on the speedo. If the computer uses speedo for management, it changes things and any benefit may be lost to the computer being confused. Also, use a GPS for milage/fuel use as the larger sized tires change odo as well. Larger tires usually cost more than stock sized so savings in fuel cost may be lost to extra cost for tires. It may not be noticable for city driving, but on hwy driving may suffer from greater wind resistance from wider/taller tires. Best bet... get 48" steel wheels (like on old prairie schooners) about two inches wide. Low rolling resistance, low wind resistance, never wear out, looks good if you get the wooden spokes painted in birght colors.