Originally Posted by lam007
So since I have 16's they are a little smaller.
Kind of yes. the rim size will make every tire different sizes. even though you are still getting a 255 with a 70 series, it will be smaller than a 17. thats why you will not find a 35x12.50R16. bc you cant really get a "true" 35" tire from a 16" wheel. some will say differently bc i believe you can get super swampers in 35x12.50x16, but it's not a "true" 35.
Sizing of tires isn't very concrete. that's why it's pretty hard to explain to others who don't know a whole lot about them. I go through having to explain sizing to people everyday, and they always bring up different points. its best to not think to concrete on the measurements of the tires. a tire size may vary from company to company and possibly from model to model with in the same company.
its best to thing of sizing generically. 255/70R16 will be roughly a 31".
here is a link that I use a ton when showing size comparisons. i think this will help you out. type in current size then your old size and it will give you measurement comparisons and all that good jazz.