Tire messurments.

[Ender2664]

So i was always taught that tire messurments worked like a 235/55 R 16 tire was 235 mm wide, and the side wall was 55% of 235 mms. That would mean that my side wall should be about 129 mm tall, and last time that i check the conversion from mm to inches was 25.4 mms in 1 inch which would mean there should be about 5.1 inches of side wall on my tire. Well i went outside and measured my tire to test this and my tire has 4 inches of side wall if that!
Would someone like to explain to me how this works?
BTW i measured my rim and it was exactly 17 inchs and it is the stock rim, so would someone like to tell me the proper way to measure a rim?
And lastly i was doing this because i am looking into getting new rims and i wanted to see how much higher my car would be ridding if i had 17s or 18s coupled with X side wall. Thanks for you input.

[FunkZ]

You're not counting the inch or so of tire bead that is sitting down inside your rim behind the lip. That lip is what is also throwing off your rim measurement, you have to measure the inner rim, the part that the tire actually goes around.

http://www.slponline.com/inst/80090.pdf

If you go with a larger diameter wheel you use a shorter sidewall tire to compensate. This is referred to as plus sizing.

http://www.tirerack.com/tires/tirete....jsp?techid=25