As I am going to Re-convert (just axles) my Van using Superduty axles and mounts from Ujoint Offroad (Chris is a big help, he posts here and other Forums as Ujoint) to deal with the added weight of the bumper, winch, and snowplow. I am planning on using his 8" springs to give me 4" more lift and to allow for 35" tires. I started looking for easy Ratio Calc's to make life easier.
and this chart makes it easy. It uses a 1:1 trans gear and 65MPH (but it is easy to figure for the OD) so by looking I can find my current tire and ratio and come up with 2809RPM at 65, to change my MPG a little for the better with my planned tire size (35") I should swap to 4.11 (4.10) ratio to get the RPM's down to 2493RPM at 65.