All about gas mileage now.

[RaysK20]

I have a 73 K20 with a semi tired goodwrench 350. It currently has a edlbrock 600 carb and intake, HEI. dual 2 inch exhaust and stock manifolds. Its going threw a th350/205 trans and case, gears are 4:10 and tires are 235/85/16.
A to B mileage on the highway at around 65 mph I get right around 11 mpg. My wife is freaking out because it seems to take a 20 dollar bill in gas to run to the store. My plan of putting in a 700R4 is in the works and I am gathering up the needed parts for a complete swap. But I also want to redo a motor and replace it at the same time so the truck is not down forever.

I have a few engines to choose from, I have a 283 and a 327 sitting in the shop as well as the 350 currently in the truck. I plan to use the above carb and intake etc. on whatever combo I come up with. I dont hammer the truck in the mud or tow anything much more then a hay trailer once in awhile. The truck is more just your daily driver type. I am really giving the 283 a hard look as it seems like less cubes to feed may help. But I have searched and come up with different opinions such as more cubes will work less hence better economy. I know I wont see 20 mpg but would like to twist at least 16 out of the old blue ox.