Jbauer

My understanding is that the aspect ratio is what determines the wheel gap from the wheel to the fender.
Right now, i've got 245/45/17...if i were to go with..say like 315/35/17..wouldn't that mean that i would have MORE wheel gap?

Just wondering..

STLBlackGT

Wheel size doesn't really matter. What fills the gap between tire and wheel opening is the diameter of the tire. It wont make any difference if you have 16,17 or 18's it just depends on the tire size. The 315 you are talking about is muuch taller than the 245. You should be able to go to a tire web site and find the dimensions on both sizes and that will show you how much taller it is and how much it will fill the gap.

Jbauer

wait, the 315 is taller? I thought it would be Wider..
which of these would fill thegap between the wheel and the fender better:
315 35 18
or
245 45 17

ponysnake99

315 and 245 are the width of the tires in mm, i think STLBlackGT got that confused with height. the aspect ratio is nothing more than a percentage (tire height/tire width), so basically:
315 x .35 = 110mm
245 x .45 = 110mm
so there is the same height in tire on both wheels. the 315 is only wider due to its tire width and would fill the gap between the inner fender and wheel but not the outter fender and wheel, that would come with ahigher aspect ratio or a set of lowering springs. as for the 18 vs. 17 it will come down to price andlooks.in the end the 18" will fill the gap better because they have similar tire heights but there is a 1" advantage over the17" wheel in overall diameter, but 18" are more expensive per tire than 17's, alot more.

Chubstang

to find the differences in the sidewalls of the tire lets say for 245/45/17, the sidewall is 45% of 245mm's so
.45 x 245 = 110.25mm and for 315/35/17 it would be .35 x 315 = 110.25mm, so the sidewall would be exactly the same. The aspect ratio is the percentage of the tread width.

or you could use this tire size calculator I just found http://www.miata.net/garage/tirecalc.html

Jbauer

Ohh i get it, sweet.

SO- a 285 40 18 is TALLER than a 245 45 17, am i correct?

I saw the 285 40 18s next to my car and they looked to fill the gap quite nicely.. would that be a good size to run in the rear?

isjoining

yeah its taller, and overall diameter with 285/40 18 will be 1.3" bigger than stock and your speedometer will read 5% too slow.

tbirdscwd

You have to remember the the aspect ratio times the width gives you the sidewall height. You have to take into account rim size. It does matter. The overall diameter is what you need to look at not anything else. The aspect ratio is just a piece of the puzzle. You need to figure out what rim size your going with and then find a tire for that rim that will give you the desired overall diameter. You can find a tire size that fills your well gap regardless of rim size.

Rule of thumb if you increase the rim size, the aspect ratio needs to decrease to maintain the same overall tire height.

TIRE HEIGHT/overall diameter= width times aspect ratio times 2, then add the rim size.
If you don't want to do the math, here is a calculator. http://www.1010tires.com/TireSizeCal...?action=submit

Jbauer

thanks for all the info! so really, everything else is BS..all i need to know is the Diameter.
Will i be safe running 285 40 18 in the rear?
i like how they fill up the gap...

tbirdscwd

You should be safe with that size as far as fitment is concerned. I personally would recommend 295/35/18 on a 18x10. It will give you a more agressive look and better traction. They are also over a half inch taller than your stock tires to help fill up the wheel wells. The 285/40/18s are 1.5" taller than stock, I think thats a bigger size increase than you realize.