255/40/18 or 245/40/18 tires?
 

My local dealer is really recommending I put 255/40/18's on my new 18 x 8.5 wheels, but I am having doubts. I don't want to tire to extend alot out past the lip of the wheel, and I am wondering if I shouldn't go with a 245/40/18. Any suggestions?

I have 245/40/18 on my rims (8 x 8.5) and they're fine. They don't extend a lot beyond the rims, but they look good and beefy.

255/40 is the widest street tire you can safely put on an 18x8.5 rim. That's what I have. Whether it sticks out depends on the amount of camber you have. I use -1.0 front, -1.5 rear and the tire fits nicely on top, sticks out slightly on bottom. Wider is better; even if it stuck out lots I'd still run them for increased traction.
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Mexico Hotels

traction=255
better looks=245.

either way, youll be okay, i have the 245/40s on mine and love them but im about to switch to 275/35/19s for the rear and 255s for the front

Can I put 245/40/18 on the stock rims?

Yes - that's my setup currently

Is that a lower profile tire? Will my rim be more vulnerable to curb rash?

A wider tire will help protect from curb rash. The rubber will stick out beyond the rim and give it some protection. I just depends on how hard you plan on hitting the curb :D

on 8.5" wheels, you can put 265/35's and have it look and drive great. The 245's fit well on the stock 8" rims.

lol

What I meant is....won't there be less sidewall? Will that make for a harder ride.

I'm looking into the Kuhmo SPT's

I'm looking for a the best tread wear.....how long will these last.

You can't have less sidewall since that would decrease the diameter of the tire. The 245/40/18 is very close to the diameter of the stock wheel.

So 40 is the aspect ratio of sidewall compared to width? The smaller the aspect ratio, the wider the tire in relation to its height?

Yep.

heres the link to a great little wheel and tire calculator http://www.miata.net/garage/tirecalc.html , now keep in mind, this is not 100% accurate, there will be almost 1/10" difference between different brands/styles of tires.

Thank you, very nice indeed!

lots of people here run 245s on the stock 8" wide rim. So if you add half an inch to the rim you can probably add 10-20mm of rubber.

255 is the widest *I* would put on, however, just because tires start to become more expensive than what I can afford as they get wider than 255

On what expertise do you base this claim on? I don't claim to be an expert--that's why I asked the folks at TireRack and those are the recommendations they gave. Tires are too important a safety item to risk on uneducated guesses.
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BRITISH COOKING

On the expertise that 1 inch equals approximately 25.4 mm
So half an inch would be about 13mm

So I guess you could call that 3rd grade expertise?

In other words, no expertise at all. Thanks for clarifying that.

You probably didn't know that a 225/40-18 tire's contact patch depends on the size rim it's mounted on, or why. Example: that tire mounted in an 18x9" rim is 0.4" wider than if you mounted it on a 18x8" rim. Take the converse of that and compare a 225/40-18 and 245/40-18 tire both mounted on a 18x8" rim. According to your math the 245's contact width should be 0.6" wider when in fact it is only 0.2" wider.
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Wendie 99

So you're saying that even though people go 20mm wider on their stock 8" rims you're afraid to go just 10mm more with a rim that's 13mm wider?


Even if you don't think of it that way...

I've got a tire on an 8" rim. You give me 13mm more rim and I will put 10mm more rubber on it and be perfectly safe.


Didn't legokcen say he's runnin 265 on an 8.5" ?

Like I said, no expertise at all. What's worse, you don't seem to comprehend what I've said in my previous post. Try arguing your point with a TireRack tech; they get paid to educate people about tires.
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DOMINICAN REPUBLIC COOKING

don't you think your being a bit critical? your talking contact patch and he is talking sidewall markings.

seems like your bashing him for recommending what you have. mike is giving a rule of thumb 8 + .5" = 8.5 rim, 245 +10mm = 255 tyre. he might say up to a 265, but some people do use this fitment, just like drifters stretch 225's on 9" rims. These fitments aren't what the tyre fitters association (or whatever governing body dictates sizes in your country) endorse but people use fitments outside these all the time and get away with it. This isn't an endorsement of using incorrect fitments but they can work with no apparent issues for some people.

im putting 245/35 on my 9.5

for example..my cousin has 215 on 9.0.. but hes more concerned about drifting ( looks crazy though)

it all depends on the look and purpose.. simple!

i ran a 275 race tire at laguna seca on borrowed 17x8" rims, and nothing exploded. i also seem to remember autocrossers showhorning 255 width tires on a 5.5" neon rim.

arent some people running 275 width on the stock rim in B-stock this year on rx8's? i am not aware of any unseating failures not due to extremely low pressure.

of course, it is best and safest to keep the rim and tire width about the same. mazda i am sure did a lot of testing to come up with the 225/8" rim ratio. however, i am going to a 245.

james

Thanks for injecting all those extreme cases guys.

I can see I was being pretty conservative in my assumptions. I could never imagine putting 275 on an 8" wide rim though!