One other consideration for gas mileage is the rear axle ratio and tire size. Numerically high axle gears won't be as economical as lower numbers in the same vehicle. Low profile tires have to turn faster to maintain the same road speed as taller tires. There is no argument that something around a 26.5 inch tall tire spinning off of a 3.73 or stiffer rear gear will give you impressive acceleration from a stop light but mileage would be better with a taller tire and a final drive under 3.00, whether you get it directly or from an overdrive trans. The overdrive trans gives you the best of both worlds, enough gear to launch the vehicle and economy at cruising pseeds. If an overdrive isn't an option for you then you have to decide how the vehicle will be used and which way to go with gear and tire combos.
Although the 292 seems to have a reputation for poor mileage compared to a 250 you have to remember that the bigger the horse, the more they eat. Another factor might be that the stock head design works better on the 250 than on its bigger brother. Sure a 283 to 305 V - thingy almost always gives better mileage in the same vehicle but the efficiency of the head design, intake and exhaust systems help.
