295 40 ZR 18s on a stock rim?

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Dec 2, 2006 | 03:32 PM

#16

John Romano

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Quote:

Originally Posted by Redridge

here you can read about the 4% rule. http://www.pca.org/tech/tech_qa_ques...3-66F33F9F8468}

You shouldn't deviate the front and rears more than 4% of each other. They will confuse you ABS, PSM and speedo setup. These cars are design as a 25" diameter tire, the farther you stray from it, the less dependable your braking, handling, comfort etc will be.

Here is a tire size calculator and see for yourself. http://www.miata.net/garage/tirecalc.html

Thanks!!

295/30/18 = exactly 25"

Cool. I'm putting them and seeing whathappens...

Dec 2, 2006 | 04:33 PM

#17

996Carrera

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I had my car also serviced at the Porsche Stuttgart factory while having 295/40zr18. The car had a full check over and nothing was metioned about the tire.

Dec 2, 2006 | 04:54 PM

#18

996 MBA CFA

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how much did you pay for your service?

i had mine checked by a german guy named Stuttgart.

Quote:

Originally Posted by 996Carrera

I had my car also serviced at the Porsche Stuttgart factory while having 295/40zr18. The car had a full check over and nothing was metioned about the tire.

Dec 4, 2006 | 11:01 PM

#19

teflon_jones

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Quote:

Originally Posted by John Romano

I would like to know what that reason is, and does fluting it necessarily mean death? My car has 285s; parked next to the car with 295s there was a very noticeable difference in tire width. It gave the car a more aggressive look that was very appealing to me. You are spot on with the perceptible difference in grip, so really, the point is moot. Naturally, Iâ€™m concerned about safety. Iâ€™m hoping to find someone who can give me a factual explanation as to if and why such an increase in tire width could prove lethal. If the difference is merely scavenging seconds on a racetrack, then Iâ€™d definitely go for the wider tires when the current tires need to be replaced.

A 295 is only 10 mm wider than a 285, or 3.4% like I said. So when you say there was a very noticeable difference, I suspect it was mostly due to a wider rim on the car with 295s versus the car with the 285s. If they were running the same width rims, then I suspect the difference was mostly a trick of the mind.

It's much more dangerous to run a tire that's too narrow than one that's too wide because a narrow tire won't have adequate pressure on the bead to keep a good seal under lateral loads. The main issue with running a tire that's too wide for a rim is that the sidewall of the tire won't be straight up and down like it's supposed to be. It'll bow in from the tread to the rim. This will create stresses on the sidewall that it was never meant to withstand, and potentially a buildup of heat. Under normal everyday driving, it will probably be fine, but if you like to drive your car aggressively for extended periods on public roads, you're putting yourself at risk.

I don't have any quantitative results to back this up, just my general engineering knowledge and experience. This is more of a philosophical argument to me since 99.99% of the time you'll be fine with a 295 instead of a 285 on a 10" rim. It really comes down to the fact that it won't give any real benefit, and IMHO, looks really dumb when I see a car with a tire that's too wide for the rim. It makes me think that the owner doesn't know cars.