I will correct myself to say:

For a 245, to maintain original tire diameter of approx 25.6" - 25.7"
(I've seen both as actual diameter spec'd by current manufacturers)
using 15" rims, you need 205/65-15 or 225/60-15.
You will be reducing the sidewall dimension by 0.5 to 0.6 inches, not much.

For a 16" wheel you will need 205/60-16 or 225/55-16 tires.
You will be reducing the sidewall dimension by nearly precisely one inch.

So, I'd still prefer a 16" wheel (duh) if I could get it past our budget.

17" would be interesting but that would pretty much require using FWD Volvo wheels with spacer adapters, or non-Volvo wheels. Either way, the price goes way up.

I've made some revisions to the calculator spreadsheet at
http://home.comcast.net/~brucepick1/tirecalcs245.xls
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DAMHIK: Don't Ask Me How I Know - - - Sven: '89 245, IPD sways, electric rad. fan conversion, e-codes, 28+ mpg - auto tranny. 500 mi/week commute. '89 245 #2 (wifemobile). '90 244 (spare, runs).