On my 74 my engine seems to guzzle the gas as if it were a 302. Maybe it just needs a tune up. But anyways, I'm sure the MPG on the 6 should be something like 22-25MPG range. If you need anymore 6cyl info, go to www.fordsix.com
I get low to mid 20's with my 200 in my '72 daily beater Maverick. It's a stock '71 long block with 57,000 original miles, MSD ignition, header, Rochester BC carb from a '63 Chevy, and a '94 Taurus electric cooling fan. Timing seems to be the number 1 thing on the sixes that hurts economy. Get the timing dialed in, and make sure it's not running rich, and you should get decent mileage. If it dosen't already have it, put some sort of electronic ignition in. Change your spark plugs every 15,000 miles.
jamie, isnt yours a stick also? i know my three speed manual 200 71 got about 22 when i was commuting in the cali S.F. south bay which is awesome for STOP and go (lol mostly stop)...