Car & Driver Plus-Size FE Results
 

Car & Driver May 2010 issue did some + size wheel testing.
Results
15X6in 195/65r Weight 40lb 23.3 mpg
16X7.5 205/55r Weight 46lb 22.9 mpg
17X8in 225/45r Weight 48lb 22.8 mpg
18X8in 225/45r Weight 51lb 21.9 mpg
19X8.5 235/35r Weight 54lb 21.1 mpg

What car?

Base Vw Golf with auto transmision
 

Well, sure, wider tires are going to use more gas (though it's always nice to see it in print). Let's see a comparison test of 19"x6" tires against the stock 15"x6".

Do they even make such a tire? They were trying to keep the OEM diameter on this testing.

Doesn't bode well for me
 

Ha, sounds like the nail in the coffin for me going to a larger (and notably wider) tire on my Jeep when these wear out.

It depends on how fast you will be driving, the final drive increase my be worth the added rotational mass/wind resistance.

the 195, not ironically.. is where I stopped as well.
I have car that can go tiny 13 inch, all the way to 31 inch mudders...
that 195 for the standard gear ratios,power to weight, has abeen a long time winner for alot of machines. I have even spotted it on rwd v8 vehicles.

its that 24.5 inch math again..nobody really says why... I can be the weird one to say it has something to do with the earth.:rolleyes:

that was a obvious result. the wieght of the rim also added to the less mpg.
the REAL question is at what point is a TALLER (regardless of width) not effective.

It's all about lowering the rpm at cruse (60mph)
I run a 255x45x18 instead of the factory 245x45x18. it's just over 3% larger diameter.
I have consistantly gotten 27mpgs on road trips w/epa of 23 and averaging over 64mph.

I think that if I could find the 19" factory rims cheep, I would try for a 6% increase over the factory diameter

THe other question to concider is the extra wieght that you are spinning when you upsize. I have been told that spinning mass is actually 4x the static wieght

That depends on its rate of rotation and its distance from the center of rotation. 4x sounds plausible for wheels, though.

Axles' rotational inertia is very small because their mass is concentrated near their center of rotation. Not so for gears, wheels, crankshafts, flywheels.