** ahh yes, you are correct. recomputation shows 6.2%, which if I WERE using TCS on the track (I have tried it and do sometimes Chris) this difference should be within tolerance on a '98, right?**
I'm not sure whether you're asking about your current sizes or your proposed sizes. Anyway, there are two ways to do the math, and either way should produce the same results, aside from slight differences due to rounding. I'll illustrate both here, using your proposed sizes of 225x45/17 and 275x35/18 and the stock sizes of 215/45ZR16 and 245/40ZR17. Calculations of percent change below were performed by the Powerdog tire size calculator at

http://www.powerdog.com/tiresize.cgi where you enter the two sizes and the website tells you the percent difference.

Method 1: Compare front to rear ratios

Ratios of outer diameter of stock rear to front = 4.7 percent larger

Ratios of outer diameter of proposed rear to front = 2.4 percent larger

Difference = 2.3 percent, should be acceptable for TCS.

Method 2: Compare change in size in front to change in size in rear

Ratios of outer diameter of proposed rear to stock rear = 3.5 percent increase

Ratios of outer diameter of proposed front to stock front = 5.7 percent increase

Difference = 2.2 percent, should be acceptable for TCS.