[ByDesign]

So many good points in this thread. Good conversation!

The key thing to remember in all of this is balance. There are good and bad effects of making changes. For instance, most cars are under-geared (IMHO) so that they "feel" peppy. Going to a larger tire diameter helps that, but only moderately.

Also, as mentioned by aardvarcus, it's the total rolling size, not wheel or tire. I just changed out 18" rims with 40's (summer sport tires) for 16" rims with narrower 70's for winter. Overall diameter is virtually the same, snow traction is immensely better.

Yes, narrower tires are better (usually) for rolling resistance and aerodynamics, but worse for traction (except in snow) and usually worse for ride and much more susceptible to damage in a big pot hole. Also will decrease braking capability.

The down-sides to larger diameter tires: 1) it lifts the car causing a increase in Cd, more drag, (all else held constant). Greater ground clearance may help with parking lots, but not with aerodynamics. 2) can interfere with internal clearances (just make sure they don't rub anywhere). 3) will change some of the vehicle dynamics (like turning capability) if you're into pushing the limits ....

Changes in wheel size is not compensated by the speedo or the odo. Most cars indicate higher than you are actually going, so check that with a stop watch and mile markers on the highway. Exactly 60 mph will take 60 seconds. (Most cars read 2-5 mph more than your actual speed.) Odo's, on the other hand, are pretty close to exact with the stock spec tires. Increasing size will show slightly fewer miles, but that's not worth the hassle unless you are really crazy about exactness -- in which case you should use a GPS to measure miles instead of the odo. (As a side note, your fuel econ measurements will be difficult to compare if you're using the odo because of the error you'll introduce with the change.)

Finally, I strongly suggest you don't mix and match sizes. Unless the car was made specifically for it, or you specifically want some other effect of size (think 60's muscle cars and dragsters) I'd keep them the same. Makes lots of things less complicated like rotating tires for best life and uniform wear, flat changes, spare tires, etc.. Increasing tire size will change the vehicle dynamics anyway, but making them different sizes may change more than you want.

Great discussion by everyone. Thanks!