Let's assume for the sake of calculations that you now have Goodyear Wrangler RT/S P235/70R15, which are 28.2" tall -- and you wish to go to their P265/70R17 32.0" tall series (Goodyear doesn't list a 255/70R17 online at http://www.goodyear.com/cgi-bin/tire...mpleselect2.pl
The simple ratio of new vs old would have it turning 88% slower at any given speed -- which effectively changes your axle ratio by that amount.
For example: If you now have a 3.55:1 final drive ratio, your engine (and speedometer) would think you then had a 3.13:1 ratio.
In order to keep your seat-of-the-pants-O-meter happy, you'd need to change your final drive ratio by 13.5% (eg: swap a 4.08:1 gearset for your 3.55).
Y2K™ 7700 4x4
Jim - N8JG@Hotmail.com
Toreador Red, Keyless XLT SC SB 5.4L E4x4 4wDisc/ABS, 3.73LS, Skid, HD 7700# Towing, LT-245's on Chrome, Tube-Steps, Captain's, 6CD, Tonneau, named: "Nick"