monkeytronic

Since the 2.7T OE tire size is 235/45-17, the 235/40-18 size will be a closer fit than a 255/35-18 in terms of overall diameter. Oddly enough, the 4.2 OE tire size is actually about 0.3" smaller in overall diameter than the 2.7T's.

In either case, a 235/40-18 or 255/35-18 tire will fit a 8.5" wide wheel just fine based on each size's respective recommended wheel width ranges.

You just need to figure out the offset of the proposed set of new wheels in question to determine if they'll fit the car.

All the V8 C5s are wide bodies and with 8.5" wide wheels they usually run an offset of about ET30 or so which pushes the outside edge of the wheels further outboard to better fill the flared wheel arches. OTOH V6s are narrow bodies and with 8.5" wheels prefer a slightly higher offset like about ET40 which would bring the wheels closer inboard and still fit under the slimmer wheel arches.

As a general rule of thumb, for a given wheel width, too low an offset and the wheel may poke out the wheel well. Too high an offset and the wheel may hit the suspension (esp. the front upright), brake caliper and/or body. So unless the car is slammed, too high an offset can be more of a problem than too low of an offset.

It's possible to easily see the effects of changing wheel width and offset by checking with a wheel offset calculator like either 1010tires.com's basic wheel offset calculator or rimsntires.com's more advanced wheel/tire size comparer. Pay close attention to the calculated effective changes in inboard and outboard clearances and verify them by measuring the car with its original wheels installed at normal ride height.

Airbag

to answer the original post .... assuming you have the same overall rolling radius ( for example going to bigger wheels with smaller sidewall tire to have same rolling radius ..

positives .
far superior road holding , cornering and high speed stbility while maneuvering caused by less flex in sidewall. Less heat buildup in tire caused by sidewall flex, visually more apealing to dsome . allows for more high performance tire choices.

negatives .
harsher ride Tires of this type tend to be significantly more expensive. very litttle protection offered to rim on pot holes. very poor performance in Snow.

Just Me

"far superior road holding , cornering and high speed stbility"

define "far superior". Adhesion is a function of shear, which is a function ( all other things equal) of surface area. Yes a 235 18" tire has more surface area than a 235 17" tire for complex reasons. But its very small.

cornering, if not == roadholding, must include subjective components, like feedback, drift angle, etc. On track, i prefer a more natural progression from zero to high drift angle. Not sloppy mind you, but i assure you 255/40-17 ZR rated PS2s are not sloppy.

Dunno about high speed stability. On road i rarely hit speeds where my 16s (ZR, PSs) are not perfectly stable, and neither, i hope, do you.

The more time i spend on track the less respect i have for crazy tire sizes. Bigger wheels mostly accommodate bigger brakes. That i do understand.

So i really must call "gross exaggeration at best" & "misleading at worst"

Ever look at F1 tires? Must suck, huh?

G

Airbag

yeah and your tractor must fly around corners ...

go to the source like tire rack or any other source and read .. I am not getting into a debate O.P. just research tire technology

2.7Lturbo

really dude?! just me knows his ****, and you calling him out because you read some info at tire rack?! bahahahahaah and to the OP, again just go buy your "Bling" wheels and put whatevertires you want on, it will look and ride like **** regardless, you are buying cheapo wheels and i wouls assume your buying cheapo tires to match... cheers!

Just Me

baloney. explain the physics to me. I think i was quite fair in my analysis.
But you are simply wrong. Pointing me to a consumer/commercial site as a scientific reference is laughable.

in any event, people always counter with " that may be the theory, but in the real world"....

so, one of the major mags, i forget which one, finally did an actual comparison with +0, +1, +2 and +3 tire and wheel packages. They plotted:

- lateral acceleration
- acceleration
- time through slalom
- slip angle

and some other stuff.

Guess what? The high profile (15" i think) was the benchmark. Then the 16" improved/ 17" was marginal, but acceleration began to fall off. 18" was falling in both adhesion, slalom (ok, slalom was increasing, but that's bad...) and acceleration. So they confirmed that there is an optimum- at least for any given car.

Now, the tire rack is a good tire store. But they are a) writing for the public; 2) in the business of selling fancy wheel and tires, and c) not college professors. So they are not exactly the definitive source. But i also can't imagine what they would say to contradict the simple facts. Can you enlighten me?

Seriously - do you understand the basics of adhesion, slip angle, shear, rotational intertia, etc? Physics does matter.

And my points remain:

1. differences are not vast, they are incremental and small
2. bigger wheels have more mass whcih is a detriment
3. stiffer sidewalls are a benefit until they become so stiff that you can't properly damp them,
4. the flip side of stiff, the slip angle, is one of the feedback mechanisms that helps a good driver read the car and its adhesion

And finally - even if you want all this, there are trade offs in weight, ride, and damage - and they generally exceed the benefits when you go beyond about +1 on any giver car's design. Since 17" 255/40 (or 245/45) is plus 1, QED.

incidentally, funny you mention it. i am shopping for a tractor. They are a ton of fun, and very useful. About 60 hp diesel with triple PTO would be ideal.

G

Airbag

yup .. i track my 911 regularly .. but do not throw around the catch phrases like I know it all because the question was about sidewall and performance tires.. all other points are deemed equal .. so keep rambling .. and thanks for coming out.

br0127

its people like 2.7Lturbo that are the epitome of a troll.

Airbag

Yeah BR you are gonna get this in any forum . it is a bit of a pack mentality around here. Point is do some research and learn for yourself . try to get some rides in cars that have different wheel set ups . That way you will be able to tell what looks good, what ride quality you need and see how the performance charcteristics change. Don't listen to anynody here . The key is educate yourself as there are many good sources of info on the net for this type of info.

Good Luck ..

br0127

thanks dude