[thabaer]

I improved the balance on my lowered '79 by keeping 205s up front and using 245/45s on 16x8s.

As far as speedo accuracy, you can use the information on some tire mfg sites where they list revs/mile for various tire sizes. As long as your rim widths are within about 1" of what they list as the median rim size for a given tire, you should get pretty accurate predictions.

I like to keep as close as possible to the standard rolling diameter, I just used Bridgestone's data to get a set of 245/45-17s and make sure I didn't poink the speedo in my '91 200q20v.