275 or 255 on 20's..?

[dib]

Hi all - I've got a question that I've tried searching for an answer for and couldn't see anything, so here goes...

I have an opportunity to purchase some replicas in a 20x9.5 size all around, so I won't be doing a staggered setup. I noticed that everyone usually does the 275/40/20 all around but when I looked at the total tire size it's only 28.6" high.

the 17's through to 19's are all 29" tall and I was wondering how it's possible to use a smaller tire. Does this not affect the odometer and anything else at all?

I also noticed that 255/45/20 is actually the right height - 29" - wouldn't this be a better tire to use? I know it wouldn't be as wide, but ride quality is just as important to me as looks...

Please let me know your thoughts...

Thanks...