Hey, I've spent the whole afternoon searching and I think I have a pretty good idea of what I need here...but I could use a confirmation...

What size tires should I get for 18x8.5 front and 18x9.5 rear?
I'm running stock performance 18's...with 225/45 front and 245/45 rear.
It looks like I should run 245/40 front and 275/40 rear....is this correct? Is this going to cover the wheel enough??

Also, here are the offsets: +30 for the front, +20 for the rear...any idea of how that's going to look? These are Gram lights.

My last question is why go 40 series when stock is 45? I thought you only changed the series # when going up or down in rim size...

 

You are correct with your listed tire sizes. If you go wider than stock, you will have to go with a lower profile tire to keep the overall diameter as close to stock as possible. I'm not too sure about your offsets, were those the offsets that were suggested by the retailer? The front should be ok, but + 20 is quite aggressive, not sure if you will have to roll the rear fenders.

 

The reason you are going down an aspect ratio (from 45 series to 40 series) is because you are increasing the width of the tire... the height of the tire is actually a function of the width and aspect ratio - that's why a 245/45 is similar in height to a 275/40 (245 x .45 = 110.25; 275 x .40 = 110 --- a 99.77% match)

 

you can then take the number you get (ie 110mm) divide it by 25.4 (mm to inch) and find the amount of sidewall that a certain tire will have - in this case 4.33"/2 = 2.165"

it gets alot more complicated when you then get into upsizing (ie 18" to 19") - we'll leave that lesson for another day...

 

Thanks! That's super helpful. I didn't realize that it was a ratio...I thought it was a constant...as in, all 2xx/45/18's where the same height.

You learn something new everyday.

Now, the offset on the rear...+20 on a 18x9.5. I noticed that most people were saying +22 or +30 was ok...so I'm a little worried about that. Any speculation?

I'm going to go do a search on "rolling fenders"...I came across that a little bit and all I got was that it is a pain.

As you can see, I'm not a "wheel" guy...kinda lost here.

 

I think you'll be ok with a +20. I have +22 with a 10.5" rear wheel and even though it's very aggressive, I don't have any problems with clearance.

 

I think it is time for a short tire lesson:

275/35R19

275 = width of tire
35 = profile of tire which is a percentage of the width, that means a wider tire with the same profile as a narrower tire will have more sidewall height because 35% of 275 is more than 35% of 245
19 = Diameter of rim

 

Oh yeah, here is my favorite tire size calculator:

http://www.miata.net/garage/tirecalc.html

 

+20 is to aggressive and the wheel will stick out from the fenders. I have +25 on the rear. The fender rolling will depend on how much you've lowered the car.