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.