[mrein3]

The first thing I would do is verify your data. If you don't own one, beg, buy, or steal a GPS. On a highway, check your speedo against the GPS. In relative terms, the speed on your GPS is more accurate than MOST speedometers. Check it at 55, 60, and 70 (where legal). Then in town check your speed at 30 and 45.

If all positions check out, and you REALLY are getting 6-8 mpg, you have problems. Get that carb working right. Tune up the engine. New air filter.

Do not immediately jump to throwing new and expensive parts at it like an overdrive tranny or higher (lower numerical) rear end gears.

The OD in a 700r4 is .7:1 where the final drive in your th350 is 1:1. The BEST you can expect switching from a th350 to a 700r4 is 30% or 2.4 mpg - assuming you really get 8 mpg now and you ONLY drive in OD - which doesn't happen.

Swapping gears will have similar results. Going between each factory gear set you gain or loose 10% of what you get now. So if you're running 2800 rpms at highway speed with your 3:73s, and switch to 3:55s, you'll take off approximately 10% so subtract 280 r's - you'll be running 2520 rpms and should see approximately .8 more miles per gallon. Drop to 3:08s, take off 560 r's you SHOULD see around 2240 r's and APPROXIMATELY 1.6 more mpgs.

So AFTER verifying your data (check your speedo), and tuning up your engine, you need to figure out how much you drive, how much you'll save with 10%, 20%, ... better mileage, THEN decide what YOU should to to improve the mileage on YOUR truck.