Originally Posted by dcstep
Well, the radius difference will vary by tire maker and tire model, since the measurements and aspect ratios are not absolute and consistant, but if you compare a 265/30-19 to a 265/30-18 the radius difference will be .5". However, as you realize, 265/30-19 vs. 265/40-18 are very close to the same diameter, with the 18" possibly being slightly larger in diameter.
Since I plan to go to a 295/30-18 for the rears, I hope to avoid the ill affects of the larger diameter that a 295/30-19, which would lower my final drive ratio. It's funny how you can fool yourself where you might say, "The extra width of these 295s sure is giving me better traction because my wheels are spinning less." You have to ask, are they spinning less due to the width or due to lowering the FD ratio???
A 295/30-18 might well have a larger diameter than the stock 19s, but it'll be at least 1" less than the same size on 19s.
Nice summary, thanks and agreed.