Philip,
Raise the rear tires off the ground, shift to neutral, rotate driveshaft and count how many times you have to turn it to get the tires to rotate once. Put a chalk mark on the tire to make it easier to watch. Make sure both tires turn, or the only tire turning will turn at double speed of the ratio.
As for gearing, I am running a 3.07 ratio in a truck with a 305/350 combo. It had a 3.9 previously. The rear end was free, or I would have gone for a 3.5 or 3.3. It is definitly no rocket ship, but it gets much better mileage and fits my driving style/conditions better.
