I owned a '74 GMC one ton dually flatbed with a '62 292 and a 4:11 rear. I got around 12-14mpg loaded or not, didn't seem to make any difference.

60 mph was comfortable top speed, the stock 292 does not like rpms much above 2500. As I recall, I'm too lazy to look it up, torque peak is between 1200-1600 depending on the year. Not surprisingly, this is a big truck engine designed for low speed pulling. Friction losses at high rpms with this engine are high. It is hard to lug this engine at normal highway speeds in a light vehicle. You have to learn not to push the throttle more unless the truck will accellerate. When you reach the point of no more accelleration you are wasting gas by using more throttle.

All these observations are based on a stock engine with a single barrel carb.

I did have headers on it because the stock manifold cracked, it does not seem to be a very good design. I may have overtightened the manifold bolts, very easy to do. Headers last forever and make the most wonderful sound you ever heard, especially at idle or slow speed. A nice rumble.

You may get 20 mpg with a light weight truck but I doubt it.

'31 Chevy 1 1/2 ton,
'42 GMC 2 1/2 ton 6x4,
'52 GMC 2 1/2 ton 6x6,
'53 GMC 5 ton