I just picked up a 1995 Mustang GT, and this is going to be my daily driver.

It is currently fitted with Pirelli P235/55ZR17 98W M+S PZero Neros, but I'm going to be facing some Michigan snow this winter, so I want to put these away in a couple of months. In their place I want to get some all-season/snow-tires and have them mounted on cheap, plain rims

The manual is telling me:
"For your Mustang GT and Cobra models equipped with 17 in. tires, to achieve the best traction during snow and ice conditions, purchase 245/45ZR17 mud and snow tires."

What makes this the best size? ...and what range is acceptable for this application?

Can I use a smaller diameter rim, and get tires with a wider sidewall?
Should I use a tire with a smaller circumference to allow for ice buildup on the wheelwell?
How about a narrower tread?
And, of course, what choices will keep the price low?

I checked with the dealership, and their parts dept. says 15"s, 16"s, and 17"s may be used.

I can also use a tire that makes up for the difference, so my speedometer will not be affected.

I think I'll look into some 15"s.