I have a 51 Chevy truck,stock 261 with a Holley 2300 series carb , 3.40 gears and overdrive.
The trucks gets about 20 mpg at 65-70 mph, goes up to 22 mpg if driven at 55. Around town it get 16-18.In OD the overall ratio is 2.48, the engine turns 2100 rpm at 70.This still give reasonable engine vacuum at cruising,14 inches which helps with fuel atomization as does a heated intake.In a hilly area, overall ratio of 3.08 might be better as mentioned above.
The frontal area creates a lot of drag on these old trucks, the weight is not a big factor on the hiway unless you're ramming up hills all the time.
