Tire size & gear ratio calculators

[swflastang05]

I need to replace the gears in my 8.8 since there's a lot of slop in them, I don't think they were installed properly last time. In any case I just switched to a 4r70w from a 5r55s and currently have 4.10's. I've been experimenting with some of the gear calculators to see if I should go with 3.73's this time and they all require the tire size to perform the calculations. I'm running Hoosier 275/60/15 DR's which are advertised to be 28x10, However when you measure them they're actually closer to 27" tall, even 26.5" with lower air pressure. My question is should I be using the 28x10 advertised size in the calculators or the "real" size of 27" since it does make a big difference?

 

[swflastang05]

I was thinking maybe the tire diameter increases with speed, like the dragsters do but on a smaller scale of course?

 

[csamsh]

I was thinking maybe the tire diameter increases with speed, like the dragsters do but on a smaller scale of course?

You would be correct, but not as much as you'll see with a bias-ply.

To be safe, underestimate on the tire diameter, then you won't have to make an extra shift if the tire is actually taller.