Chevrolet Repair: old truck efficiency, carter afb, speed surface

Question
I recently purchased a used P.O.S. 1981 chevy c20 w/ a 350/th350 combo.(it was cheap)  I'm trying to get this truck to run as efficiently as possible.  It is relatively stock, aside from a Carter AFB 9635sa carb, and what I believe to be really steep rear-end gears.  I rarely carry any significant weight in it.  What is the lowest numerical gear ratio I can go with without having to "creep" away from stoplights?  I plan on having tires close to stock height, and I drive mostly freeway and "higher speed" surface streets.  (usually 45mph posted speed limit)

Answer
As far as gearing you would want to find out what it has now which you can tell by pulling the numbers off the rear axle and calling you local dealership to see if they can tell you what it has stock. But with that combo you will want to go with like a 323 or 342 gear in the rear. But in any case a 350 with a carter AFB isn't going to get the greatest mileage anyway. The Carter AFB is considered one of the greatest off road carbs built for power and anything built for power isn't going to get the greatest efficiency.