Will The Overall Wheel Diameter Change When..?

[so_tred]

This is probably a really stupid question but anyway..

So I have an R34GTT which come with 17 x 7.5 wheels standard (as we all know..) my question is when I fit new wheels (Front 17x8 - Rear 17x9) is the overall diameter of my wheel+tyre going to change..?

Someone said it would, hence my car will sit higher etc..but my thinking is, if its the same size rim (in diameter - 17") why would the profile get larger..?

[nisskid]

depends on the profile of the tire. the thing u may notice is that if u go from a 205 tire to a 235 tire both with a 45 profile for example because of the extra width of the rim, the height of the sidewall will change as the profile works with the tread width, eg, a 205/45 will mean the sidewall is 45% of 205mm, so 45% of 235mm will be higher, hence creating a larger overall diameter. but if u keep the same tire with the same dimensions i dont see why u would see any increase to the total diameter.

[Duncan]

well basically its not just the rims that matter, its the combination of rim and tyre (width and profile).

you will be to find a tyre with the same overall diameter as your old ones - just tell them what the old size was eg 235/45/17 and they will be able to choose something close enough eg 265/40/17

[nisskid]

yeh its not hard to do the maths, say u start with a 205/45: 205x0.45 = 92.25, 92.45/0.40 (the next profile down) = 231.125 round it off to a 235, so a 235/40 will be roughly the same as a 205/45, or say u want to run a 255, 92.25/255 = 0.36 or round it off to 35, so a 255/35 should be about the same height as a 205/45 etc etc

[n15m0]

screw all the math! Use the tyre calculator...

http://www.1010tires.com/TireSizeCalculator.asp

[nisskid]

^^haha yeh gotta love that site, but that only works one way, u can use that to confirm sizes, but if u want to work backwards u have to do the maths urself, then u can go on there and put in the 2 tires to confirm its all good.

[n15m0]

The calculator is useful for determining the difference in overall rolling diameter compared to stock...so you will need to know what your stock wheel and tire combo first before sticking in any different combinations. In QLD, legally you cannot increase the diameter by more than 15mm and if you stick within 15 mm, your speedo difference will be negligible.