Derek,

I put into the Calculator a 16" rim with 235/80 tires (about 31"
diameter) and a 6.13 final and got 73 mph @ 4000 rpm. With 25" tires
(185/78x14) and a 4.86 final 4000 rpm I got 75.7 mph. Using my other
program it would take a 32" tire to get to 75.9 mph. Is that what you
are looking for?

Scott 

> Cool! But what I want to do is find out what size tires the factory
> 6.13
> gear ratio gears would cause us to purchase to have the van have the
> same
> characteristics as the 25" tires that are original to the 4.86 gears.
> 
> I couldn't find out how to do this with the gearing calculator, so can
> you
> help?  ...or anybody else, for that matter?
> 
>