I have purchased a pair of 18"x8" chrome IROC wheels for the rear of my car, and need to figure out what size of tire to get so that the sidewalls don't look out of proportion to the 245/50ZR16's up front. Don't want skinnys out back with fattys up front.

 

I should add that I'd be willing to consider different front tires too, if it would make it easier. I don't really want rubberbands for sidewalls though. My current wheels are chromed original IROCs 16"x8"

 

245/50/18

Will have the same size sidewall as the fronts. Pretty simple eh? Are they available that size? I have no idea but I kinda doubt it. Its easy to figure sidewall height, take the first number and multiply it by the second, then divide by 100. Thats sidewall height in mm.

Probably something in the 245-255 and 40 series is what I would guess you will find. I dunno about putting a 275 width tire on an 8" wide rim, might work ok. Only problems I can think of is the tire fitting in the opening (youre adding 2" of diameter if you keep the same sidewall height) and your speedometer is gonna be off unless you ran rubberbands.

 

A 245/50 18 will be something like a 28" tire. There's no way you're going to get an 18" tire with the same amount of sidewall as the front tire and fit it into the wheel well. If you get the same overall rolling diameter (26" is stock), you're going to have a whole lot less sidewall. Mr. IROCZ can tell you what size tires he's got on his 18" IROC wheels. I believe that he still has a 26" rolling diameter.

 

you can fit a 28" tire with stock suspension, but you'd have to go to a higher (numerically) gear to cancel out the height change of the tire.

 

Your new size should be 245/40R18, its very close in diameter to your 245/50R16s.

 

Get 275/40/18's, they are a little bigger diameter so you can get the correct big, little combo look.

 

As mentioned earlier by Madmax, you will need a 245/50/18 in order to keep the same sidewall which is ~4.82 inches. However, with this tire size the Radius and Diameter of the 18" tire is more. The Radius is ~12.82 inches with the 16" and ~13.82 with the 18". The Diameter is ~25.65 with the 16" and ~27.65 with the 18". When all is said and done, w/out recalibrating your speedo, it will read ~7.8% too slow.

In order to change tires w/out messing with the speedo, you would need a 245/40/18 .... which I believe someone mentioned in an earlier post. The Radius and Diameter of this tire is almost the same at the stock 16" tire and the speedo would only be ~.28% too slow which isn't a noticeable amount. However, with this tire you will have approx. 1 less inch of sidewall.

In my opinion, I would go with the 245/40/18 (if they make it) because you wouldn't have to recalibrate the speedo and the tire is the same width as the stock tire so it wouldn't look like you had skinnies up front and fatties in the back. I think it'd look good with 245/40/18s in the back and 245/50/16s up front.

 

18x8 chrome IROC rims
Cool! I got 16inch IROC rims (gold). What do 18 inch IROC rims look like on your/our cars. Post a pic of them, pretty cool.

 

245/50/18's will be the exact same sidewall height, but when you compare it to the 18 inch rims, the sidewall will seem wayyyy smaller than the 16s, and youll end up with a normal appearing tire in front and the rubberband look in back. its going to bevery difficulty to get the proper look, and to do so youll need to INCREASE sidewall height in the directly proportional amount you increased the rim size by, i.e. 16 to 18 equals a 112.5 percent ratio, the tire should be 275/50/18, and while that tire is rather large to fit an 8" wide rim, you might fit it. im not sure if you could find a 50 series tire for an 18 though. and not to burst your bubble, but a 345/40/18 would be the requisite 40 series tire for the proper look, although it will not fit an 8 inch rim. a 305/45/18 might work, although youll probably end up with a rounded sidewall and that just wouldnt be good. sorry to say it, but the only look your gonna get on an 8 inch rim is the rubberband look, except maybe truck tires. but i couldnt tell you for sure. but for any of the combinations above, youll definately need to recalibrate your speedo, but you can get that done at a number of shops and that wont cost you much. my advice? buy two more rims and match all the way around, but money may be a constraint, so good luck.

 

no way a 275 can fit on an 8 inch rim... if you do, i wanna see

 

I have 18's on my car and I have these tire sizes: Front tire is 245/35/18: Back is 275/35/18. I dident measure it but when I was buying them I put them right next to each other when they were both off the car and they were about perfect. Only difference would have been because of worn out tread. My spedo is still right on and I dont rub at all ( and my car has a 2" lowering kit on it ) :also my front springs are cut a little too

 

Thanks for all the help guys. My local tire shop has been useless. You've addressed what I'd need to do if I kept the same front tires, and matched the rears to them. How about if we looked at it from the other direction, if I were to choose a properly fitting 18" on the rear, then what size would I need on the front? I'm starting to understand better that I may have no choice on the skinnier sidewall. I really want the big and little look, so it isn't that I can't afford 18s all around, its that I don't want them.

So, I guess first up, what size tire for to properly fit an 18x8? Then what size to match up in 16x8?