Originally Posted by SRT SIX
I could be wrong, but it is my understanding that the treadwidth (285) the aspect ratio (30) and wheel diameter (19) is relatively constant from brand to brand very unlike the "suggested" treadwear rating. It is possible to measure the treadwidth a little different based on how the tread pattern wraps over the edge of the tire but overall the ratios are fixed.
Another tire comparison calculator can be found here
Tire Size Calculator great for converting tire sizes.
Comparing what is our standard 255x35x19 (tire #1) rear tires with a 285x30x19 (tire #2) yields the following in inches.
Sidewall height #1 3.51 vs #2 3.37 for a difference of -.14
Section width #1 10.04 vs #2 11.22 for a difference of +1.18
Tire height (diameter) #1 26.03 vs #2 25.73 for a difference of -.3
Tire circumference #1 81.77 vs #2 80.84 for a difference of -.93
Tire revolutions per mile #1 799.08 vs #2 808.39 for a difference of 9.31
Speedometer reading #1 at 65MPH will equal #2 64.25MPH
Overall close enough for me for a nice gain in surface contact.
You are correct in stating that the aspect ratio is a bit more consistant through brands than the treadwear ratings are. But you still should do just what you've done by comparing each size/brand side by side.
You can tell if 1/8" or 1/4" will be a show stopper one way or another.
And I fully agree with looking at alternate brands to get the best bang for the buck. These tire aren't cheap! I still have my original PS2s all around with only 13k on them, I'm a very easy on my car and they appear to be about half gone. And yes, one of my fronts seem to be developing a slight bubble....have to keep an eye on that, and I definately CAN'T afford to get PS2s again.
Cheers
Paul