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Discussion Starter · #1 ·
It's actually pretty simple..

Compared to Stock
TLS/CLS = 215 * 50 = 107.5mm
TLP/CLP = 205 * 60 = 123mm

Total tire height = * 2

But you have to consider the diameter of the wheel as well, since wheels are given in standard, but tires are in metric, it gets a bit confusing.. so just convert everything to metric..
given 1 inch = 25.4mm

17 inch = 431.8mm + 2(107.5)mm = 646.8mm total wheel hight
16 inch = 406.4 + 2(123) = 652.4 total

225/40/18
just means:
225mm * 40% = 90mm sidewall height

18(25.4) + 2(90) = total
457.2 + 180 = 637.2mm

so 225/40/18 is 9.6mm shorter than stock TLS/CLS height

for 225/45/18 calc
225 * 45% = 101.25
457.2 + 202.5 = 659.7 total height.

224/45/18 is 12.9mm taller than stock TLS/CLS

The same formula can be used for TL/CL 16 inch stock as well

D + 2(ar * tw) = total height..
where:
D = Diameter of wheel
ar = aspect ratio of tire
tw = tire width

all in metric (mm) measurements..
Here is a website that is good for calculating it for you:
http://www.miata.net/garage/tirecalc.html

***********************

Ok, now that you understand wheel/tire sizes.. lets go into how offset plays a part in it..
Okay, here it goes.. This is for TLS/CLS

Stock offset is +55 on a 7 inch wheel
Not sure about clearence, but we all know it's 4x4

Think of offset as two independant rings, one smaller inside of the larger..
the outer ring is your wheel, and the inner ring is your mounting surface.. how even with the wheel the mounting surface is, that is your offset..



0 offset means that the inner ring is exactly in the middle of the outer ring..

so our stock +55, makes the mounting surface closer to the outside of the wheel, effectively making the whole wheel tucked into the middle of the car..

by reducing the offset to +42, you are moving the mounting surface closer to the centerline, effectively making the whole wheel stick out away from the car. In this case, by 13mm. You could probably get away with it if your tire size stayed the same..

but if you are thinking of 225 tires.. which is 10mm wider than stock.. but you take half of that for either side of the wheel giving you only 5mm wider on tires.. add that to the 13mm you have already moved the wheel out and you have 18mm total from the stock position. Again, probably ok if you do not lower it..

So with a higher offset, the wheel will stick out less, or more correctly, not move from the stock position as much.. It seems that +45 is probably the lowest you could go without too much rubbing..

On a 7inch wheel, 225 is 47.2mm wider overall than the wheel itself, 23.6mm on either side.. but it seems that a skinny wheel will make the tire sidewall not perfectly flat and will cause it to bulge out a little bit.. which is fine for a lower profile..
People get wider tires to get more of the tire surface to the ground for better traction, and also to help protect the wheel from getting scratched as easily..

And if you are going to lower with Comptech, wich is 1.75 closer to the fenderwell. So everytime the springs compress, you'll even have less room for the suspension to move causing the tire to rub against the fenderwell or the bumper tab, or the shocks, etc..

With wider wheels, you have to worry about rubbing not only on the outside, but also the shocks..

with wider tires you really only have to worry about the fenderwell..

Here is an offset calculator:
http://toy4two.home.mindspring.com/offset.html
 

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NO AMG, NO CARE
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This deserves a sticky IMO :) :bigok:
 

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2,075 Posts
We should put this in FAQ.
 
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