Currently I'm running 17" aftermarket wheels on my 02 Mini S and the tire size is a 215/40/17 I am in the market for some new tires and a friend of mine has a great set of 215/50/17. My question is how much difference is this. Also my car has Koni shocks all the way around and the car sits about an 1 to 1 1/2 inches lower than stock.

 

Thats thats about 1.69 inches larger in diameter. Might look like a monster truck.

 

That's 7.1% larger than your current tires, and 4.9% larger than OEM (205/45-17) In general you want to stay within 2%. Your current tire is 2% smaller than OEM.
see http://www.miata.net/garage/tirecalc.html for a great calculator.

 

Ok, Thanks alot! I knew I could find people smarter than me. I'll stick with the size I'm running now.

 

Yep.

It's not how wide it is...

 

If you want to go a little bigger on tire size, going from the OEM 205/45/17 to 215/45/17 is a good alternative. It's a little wider (~.4") and a little taller (.35") and clears the wheel openings with no modifications unless you are lowered a lot. Since MINI speedometers read higher than actual from the factory, the taller tire will get your speedometer a little closer to actual speed. I did this and now my speedometer only reads ~1mph high vs ~3mph high (@ 60 mph) when comparing the spedo to a GPS. Having 215/40/17 tires actually makes the speedometer farther off since it is almost a 1/2" shorter dia. than OEM. With these tires you are probably going about 5mph slower @ 60mph than the speedometer is showing, although this will keep you from getting tickets.