Hi Snowman.
I moved your question to this section of the board because it was posted in General Chat.
If you look at this old thread here;
http://challenger.mpoli.fi/phpbb/viewto ... 0093#10093
You will find this handy Excel spreadsheet where you can enter your tire size and try different rear axle gears. It will tell you your speed at every rpm in every gear.
The lower the gear (higher number) the more torque you get at the rear wheels but the trade off is a lower top speed and usually (but not always) lower gas mileage.
I usually advise people to select a rear gear that allows them to cruise at their usual highway cruise speeds at the same rpm as where their engine is making peak torque.