CarHopper

Ingenieur

BHimself

The stock for our cars are 235/40/18 & 255/35/18 I believe.

To ask the really dumb question, why are the profile's 40&35 (F&R) respectively?
Does a 235(or 245)/35/18's not work with our car for some reason?

Thanks to anyone willing to explain this, I'm not amazingly versed in tire knowledge.

whoover

I won't address what "works," because that's a loaded question and I don't want to start anything right now, but the point of staggered tire setups is to have wider rear tires for traction and narrower fronts for steering quickness. That's an oversimplification, but good enough for now.

When you want to widen a tire, in general you lower the profile and go wider, which has the effect of limiting the change on the tire's circumference, which I don't think anyone would argue against being a good thing. (The debate is how bad changing it is.) The profile is a measure of "ovalness" of the tire's cross section. The lower the number, the "thinner" the donut is for the same width.

The downside is that lower profiles also are more responsive, which is why a larger wheel with a lower profile tire (a +1 that maintains overall circumference) will improve steering because it replaces mushy tire with stiff wheel. That's why you might go from 18" to 19" wheels and lower the profile of the tires.

So the higher profile up front is just a way of keeping the fronts and rears roughly the same diameter while making the rear wider.

Ingenieur

The profile is the ratio of side wall height to width
As you said it's called the sidewall profile
Or aspect ratio
It = 2 x sidewall height / width x 100

Or tire diameter = rim + aspect ratio / 100 x 2 x tire width

Since the rears are wider they must have a lower aspect ratio to have the same diameter

255/35/18
Diameter = 18 + ( 2 * 35 /100 * 255 ) / 25.4
= 25 "

It's basically the ratio of the sidewall height to the tread width
Since the rears are wider to get the same diameter as the front the side wall must be shorter, ie, 35 vs 40

zcct04

+1

zibby43

This thread might inadvertently help to create cold fusion.

BHimself

Thanks guys. I appreciate the responses.
I asked because DiscountTire put on a set of 245/35/18's on my car, but one has developed a blister on the sidewall, and the other has picked up a nail, so now as I'm trying to get them repaired/replaced, their system is saying the load rating of these is too low, so they're forcing a 245/40/18.
Sounds like DT has gotten a system update, lawyer edition this year.
I just thought it strange as they put me in the 35's in the beginning...but uninformed types like me are probably benefiting.

Thanks again.

YICS NZ

Just turn off the bloody ESP. Everyone's happy