05SDI

A little tire size 101;
The tread width only plays a part in overall tire diameter.
Lets take a stock tire off our GT Mustangs (yes they can come in different sizes) Now each tire brand and type can be slightly different over all size but this will keep us close.

Now this tire is 235/55/17. This tire's tread measures (close to) 235mm, the side wall equals 55% of the 235mm tread. One needs to use the aspect ratio, (second number) and multiply this aspect ratio number (%) with the first number and this will give you your side wall height in mm. Our stock tires have a side wall of 129mm now if you had a tire that was 275/45 it would be very close in over all diameter.
SO a tire that is 275/45/? would have a larger overall diameter then a 285/30/? or even a 305/30/? tire. Being in this example
the 275/45 has a sidewall height of 123mm and the 305/30 has a side wall height of 91mm. On a 275 tire ever 10% aspect ratio is very close to on ench over all diameter.
Yes it is confusing but it is pretty straght forward. BUT There is a difference between tire brands.... 05SDI

howyhowy

So if my rims are gonna be 18x9 that means I should be able to fit (9+4)*25.4 = 330 tires on them (not counting if they would actually fit under the wwell)?

When I got my 275's for my 17" rims I just crossed my fingers while my tire guy banged on it with a hammer till it popped in. If it had not fit, I would have been stuck with 4 new tires to display on my mantle. I wonder if the profile of the tire somehow influences the ability of a tire to mount onto a given size rim...

I would really like to have to the option of putting 315s or 330s on the back, but I already went and pulled the trigger on four 18x9s with 255s so I guess I won't know till these tires wear out.

-howy

delz05

Our 17x8 in GT stock wheels should NOT be mounted with a tire wider than 255mmwith a low profileside wall (40 or 45). It will put too much stress on the side of the tire and could lead to tire failurein impact or high stress(ie high speed turns, pot holes, curbs). If you increase the sidewll to a 50 or 55, you can use the 275's safely. While the 275/40/17's can be forced on, you are taking a chance. Better to have them mounted on 17x9 inch wheels

spdbrake

Visual Calculator.
http://www.bigcustomwheels.com/rt_specs.jsp