[jim_howard_pdx]

This is not my strongest subject, but I will give this a try. In the beginning Mustangs came with 14 and 15 inch wheel sizes. For about a year Michelin built a 15.5 tire for FORD that are un-obtanium.

16, 17, 18, and 20 inch tires are built mostly for extreme performance. The idea is to reduce the sidewall's influence upon the suspension in order to gain high speed cornering ability. That is why you see 35, 40, 45 ratios so commonly.

It used to be that all street tires were 75 or 70 ratio tires. That means if the tire was 5 inches wide, it was 3.5 to 3.7 inches tall from rim to tread.

With an 18" 35 ratio tire a 7 inch wide tire is just 2.45 inches from rim to tread. So when cornering at 100 mph, the sidewalls produce less distortion to the suspension, allowing the car to groove.

As you go up in rim size, you can retain ride comfort by going a 45 ratio if available. When you get to 18 inch tires, you are going to primarily find 35 and 40 ratio tires. But in 17 rims you will find 45 and sometimes 50 ratio tires.

A ZR rated tire has lots of stick, but don't expect it to last more than 25-28,000 miles. I prefer Z-X rated tires, which go between 35,000 to 40,000 miles. They are a little harder rubber, A GOOD THING ON THE STREET, but will not give you the great stickiness of a ZR.

Hope this helps.

By the way, if you go 17 inch rims, the tires will be much less expensive. If you want to go ZR tires, then 17 inches will be much less expensive over the years.