Opinions? Tire size vs. Fuel Mileage

[The Mad Hatter]

Ok, well i know that my 85 Golf Diesel will see regular commuter duty at 70-75mph.  I figure the stock 13 x 5 wheels/tires wont be super comfortable at those speeds, plus wider tires improve handling, and i'm a "spirited" driver.  So, my question is, how much MPG will i lose switching to larger wheels/tires.  I'm looking specifically at Sport Edition D5's, in 15 x 6.5", with 195/50R15 tires.  The wheels weigh in at 15.4lbs a piece, not sure how much a stock 13x5 steelie weighs.  

So, what's everyone opinion on this.  Better to keep the stockers or go with the larger ones?

[G60ING]

I know on my wife's 2003 TDI and my Corrado TDI we both lose about 2 mpg when we run 17" wheels versus our 15" wheels. But after a 90mph late night swerve to avoid a tractor trailer's tire that was standing up in the middle of I-95 in the wife's 2003 Jetta I determined the fat sidewalls would never have allowed me the manuver and continue on my way. So now my ideal tire has a 45 series sidewall on all my TDIs.

[Baselyne]

I have 17's on my a1 and i cant see any changes really

Id say more change in the speedo than anything

[Vanagoner]

I think width makes more of a difference.  This is from Audi-
"special size 145/80 R 14 tyres of a new design not only reduce rolling resistance on the A2 1.2 TDI, they also bring aerodynamic advantages. They are narrower than the standard tyres on the other A2 models, and are also aerodynamically optimised by fine grooves incorporated into their sidewalls."

[KTZed]

Tires with a larger OD will lower your hwy rpms and might get you better mileage