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Old 07-27-2004, 04:57 PM   #1
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What is the largest size tire that can fit on the stock 18 inch rims?

What is the largest size tire that can fit on the stock 18 inch rims, width wise preferably? (and before you flame me i did do a search and couldn't find this specific thing)
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Old 07-29-2004, 12:20 PM   #2
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245/40WR18

But you need to look at the manufacturer's specs. Now, that is just the tire/rim combo...the bigger question is tire/rim to body clearance.

I am sure others will have a more definitive answer than mine.
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Old 07-29-2004, 02:34 PM   #3
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Been discussed many times.

245/40's will fit perfectly fine. Although 245 is the largest tire you can put on an 8" rim, the tires don't have a "fat tire" look.

Popular high performance all-season tire has been the Yokohama AVS dB2's that I have and the Pirelli Nero M+S. Eventhough these tires have a higher treadwear rating and are "all-season" most people feel they actually grip better than the stock RE040's.

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Old 07-29-2004, 03:29 PM   #4
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This may be a dumb question (or one that was also discussed before :p) but by going with 245, isn't the fuel economy or power/acceleration going to be adversely affected?
I mean, the 245 has probably more rolling resistance... or is this not a significant factor? Also, is this going to be OK for the steering mechanism still?
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Old 07-29-2004, 05:51 PM   #5
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Quote:
Originally Posted by Tamas
This may be a dumb question (or one that was also discussed before :p) but by going with 245, isn't the fuel economy or power/acceleration going to be adversely affected?
I mean, the 245 has probably more rolling resistance... or is this not a significant factor? Also, is this going to be OK for the steering mechanism still?
The short answer is it ain't no big deal. The tire is a little wider but not enough to make it harder to turn. And because the contact patch with the pavement is now a little less long and a little more wide the tires might actually be a little easier to turn.

When I switched from 225's to 245's I didn't really notice a difference at all in terms of steering wheel effort.

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Old 07-29-2004, 08:08 PM   #6
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aight thx guys, yea i have a friend who can get me Pirreli Nero M+S for 60% off and wanted to see what was the largest i could get.
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Old 07-29-2004, 08:21 PM   #7
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Largest tires? 275/35-18. No rubbing lock to lock. Just ask tpryor.
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Old 07-30-2004, 06:35 AM   #8
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Largest tires? 275/35-18. No rubbing lock to lock. Just ask tpryor.
Not on a stock rim.

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Old 07-30-2004, 06:53 AM   #9
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Absolutely on a stock rim. Pictures at the link in my signature..

I just had a new set of 275/35/18 A3S04's mounted last night........

FWIW - I test fitted a 285/30/18, and it fits fine. Looks like it would easily fit on the rear, but it looks VERY close (at full lock) in the front. I stayed with the 275...
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Old 07-30-2004, 09:07 AM   #10
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well you can fit any size into any rim but the question is wheather it is safe!

It is not recommanded to fit anything over 245 to a 8" rim. The dealer who sold you a 275 to your 8" rim is not thinking your safety mate!

It is totally breach the code of conduct of a tyre saleman. If he recommands you that and did not say anything of this being a bad idea. Crash the car now and SUE HIM! You will get a new car and extra money for pain and suffering... or maybe a new wider wheel for your 275 tire
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Old 07-30-2004, 10:00 AM   #11
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Takahashi - - - -

These are Hoosier race tires, they are NEVER driven on the street. I am limited to rim size based on the rules of the sport - "....any tire that will fit on a stock sized rim..." (paraphrased, but accurate).

I have run 245/45/15 Hoosiers on a 6" rim (on the Miata) - I have 4 National Tour wins in that car.

I have the stock 225's for the street..........
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Old 07-30-2004, 11:14 AM   #12
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According to tire size calculators on the 'net, 255/40 R18 tires are almost dead-on same diameter, circumfence etc. like the stock 225/45 R18 rubber. Why do people go with 245/45 R18 then which is not that close to stock? Is the 255 size not safe anymore?

Taka, you said "It is not recommanded to fit anything over 245 to a 8" rim."
Where does this recommendation come from? Would it be really dangerous to go with the 255 size?
Maybe 3 cm wider tires would already be too much steering effort so that wouldn't be good for the car... hmmmmm.
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Old 07-30-2004, 05:30 PM   #13
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It is all due to manufacturer's recommandation... check with the particular bland you are looking for. I know that I can only go for 245/40 when I change to the S03 Protenza. It is just on the table of the tyre choices of the recommanded rim size. There is no magic answer but it is mainly for the insurance company.

For Australian law, I know that for a fact, as long as the wheel does not stick out then it is legal. But most insurance company allows only 1" bigger (ie 19") and 1" wider 9" and the tyre is not allowed to go 20mm above in the stock rim. Stupid aren't we

tpryor... that is so unfair! I guess the question did not specify passenger tyre. So I cannot call you a cheater
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Old 07-31-2004, 07:17 AM   #14
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Cool

It gets down to the point that the wider tire does not sit properly on the road with the smaller rims. The beads are pulled closer together than they should be thus pulling the edges of the tire tread upward and away from the pavement. You may actually be getting less rubber on the road using a fatter tire. Almost like having the tire over-inflated.

Anyway, check with the maufacturer for recommended rim sizes...245/40/18 seems to be a good size for the street keeping all the roll-outs and other specs close to the original tires.
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Old 07-31-2004, 11:20 AM   #15
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Quote:
Originally Posted by takahashi
tpryor... that is so unfair! I guess the question did not specify passenger tyre. So I cannot call you a cheater
Quote:
Originally Posted by Katchoo
It gets down to the point that the wider tire does not sit properly on the road with the smaller rims. The beads are pulled closer together than they should be thus pulling the edges of the tire tread upward and away from the pavement. You may actually be getting less rubber on the road using a fatter tire. Almost like having the tire over-inflated.
OK - lets put this one to bed................

Taka/Katchoo, if you look at the attached pictures of the 275's, I think you would agree that they appear to fit fine on the stock rims, and do not appear to "bulge" in the middle from being squeezed onto the rim. It took less than 15 minutes to mount all 4 of these.

Also notice that the A3S04's are worn evenly across the tread - there is 2/32" left uniformly across the entire width. These ARE DOT approved tires, so they could be driven on the street (I put 15 miles on the R3S04's this morning) if desired.

I took pictures this morning, and they also seem to be sitting properly on the road........

The point is, that all tires are different. The government mandates the "recommended rim size" for the tire manufacturers, they have no control over it.

Anyway, as you can see, they fit fine, wear fine, and look fine on the stock rims.......

I loaded the high-res versions of these (and a couple of others) to the "RX8 SOLO II" folder at the link in my signature.
Attached Thumbnails
What is the largest size tire that can fit on the stock 18 inch rims?-275_leftside.jpg   What is the largest size tire that can fit on the stock 18 inch rims?-275_vs_stock.jpg   What is the largest size tire that can fit on the stock 18 inch rims?-a3s04-wear.jpg   What is the largest size tire that can fit on the stock 18 inch rims?-r3s04-size.jpg   What is the largest size tire that can fit on the stock 18 inch rims?-a3s04-size.jpg  

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Old 07-31-2004, 11:03 PM   #16
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I put 245/40 Pilot PS2's on....comfortable size for me. I guess tpryor proved you can go bigger, But I think 245 is sweet - so are the PS2's
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Old 08-01-2004, 10:45 AM   #17
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But why are we looking at non-street tires? I don't think that is what this is about.

I am not a tire expert by any means, and I have stated that you should go by the manufacturer's recommended rim size for a particular tire, but the point is that if you put on a tire that is too wide for the rim you will have road contact, wear, and possibly body/tire interference problems.

Go to the Tire Rack web page and for each size/brand of tire they give the recommended rim sizes. You can go bigger or smaller, but on the road you may run into problems and not get the most out of the tire.

Let me see...*grabs tire iron and starts to pound on 15" wide tires* Oh yeah...they fit on my stock rims. LOL
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Old 08-01-2004, 10:50 AM   #18
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Now I am going to bed...

~dreams~ stock rims~ZzzZZzzz~street tires~ZzZZzzzz~manufacturer's recommended tire size~ZzZZzZz~~~no problems~~~ZzzZZzzzz
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Old 08-01-2004, 11:41 AM   #19
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Pilot PS2's are street tires
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Old 08-01-2004, 09:00 PM   #20
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True...but the ones I was refering to are the 275/35/18 A3S04's race tires that have been tossed into the discussion.

I am pretty sure that the question was about street tires rather than race tires even though that was not specifically stated.

Anyway, time to move on to other threads so I can rant and rave across the board. LOL
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Old 08-04-2004, 07:46 AM   #21
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Seems to me that someone should make a sticky with the largest size tire that can be mounted on an OEM wheel, & the largest size tire that will fit in the wheel well, as well as some associated discussion - this seems to be a subject of some interest around here quite frequently...
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Old 08-04-2004, 08:05 AM   #22
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The only problem with putting a 275 tire on an 8" rim is that you will increase the slip angle of the tire...

...That said, you have so much rubber under the car that you will grip more than a 245 tire on an 8" rim even though it's slip angle is lower.

Just keep the pressure of the 275 >40psi and you will be fine
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Old 08-04-2004, 09:45 AM   #23
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Alex, why does a wider tire increase slip angle?
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Old 08-04-2004, 11:22 AM   #24
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it doesn't...

A 275 tire has more slip angle on an 8" rim than it does on a 9" rim. The wider rim provides more support from distortion. More distortion...more slip angle
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Old 08-04-2004, 11:22 AM
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