Need help with Tire math

Does anyone know of a web site or a book that can explain the mathematics of tire sizing? Becasue of the non availability of decent high performance street legal tires in 15 inch rims, I have decided to go with 17s, however, I KNOW what size 15s will fit, but don't know how to convert the size over to 17 inch.....I went to a local tire store and asked if they could help, I told them that I would like to replace a 225/60 15 front and a 275/55 15 rear and the guy behind the counter placed a call and then said I should use a 215/50 17 front on an 8 inch wide rim and a 285/40 17 rear on a 10 inch wide rim. Pirelli happens to sell these sizes in a PZero tire, but how do I know if this guy is right? I'd like to be able to do the math myself to be able to understand if I could use, for example a 225/45 17 to replace the 215/50 17 on the front if I found another tire brand I would like to use instead, like Kumho. Amy help would be appreciated.
Thanks
Brian

the key is keeping the overall tire diameter equal to what you had before.
here is the formula I use:

2 X section width X aspect ratio / 2540 + rim diameter = tire diameter

therefore your 225/60/15 front is

2 x 225 x 60 / 2540 + 15 = 25.63 inches tall

Hi Brian,
The gt40 enthusiast web page has tables for all tyre sizes from 15" to 18" and different aspect ratios. try this link http://www.gt40club.clara.net/technics.htm#Wheels
cheers
Dave

Remember that the ROLLING diameter may be very important: this stuff can be found on maunfacturers web sites, or a large on line site such as The Tire Rack...
Remember also that the width is only on the specified rim for that tire as specified by that manufacturer..and often the same width tire wil be measured on different width rims by different manufacturers...a 1" rim change generally alters the tire with by 0.4"...so it is very much looking ino the fihures carefully, especially with space limitations, or comparisons between brands or types



Rob

Thanks guys, now I may be a little better armed to make a decision. Now I just have to get over the way the 17s look on the car. Maybe I shall be compensated by the looks of huge rotors and calipers lurking behind the spokes.......

Brian

This link will get you to a nice little tire size comparison Javascript application.

This website may be of some use to you in comparing tyre sizes
http://www.miata.net/garage/tirecalc.html

Tell me what size you need to compare against or match up to and I'll post back, as I have to get back home and check my charts.