what kind of mpg are the new trucks getting? I want to put a newer engine/trans into my 71

Stock 1989 GMC S-15 4.3L (Automatic w/OD)
(no AC) and getting about 20mpg average.

Hope this helps.

My '97 GMC has Vortec 350, 4l60e, and 3.42's. If speed held constant around 65-68 I can get about 22mpg. If getting on it about 18.5-20 mpg.

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If patience is truely a virtue, Then I am so screwed
1970 Chevy SWB being reserected.
1997 GMC 1/2 Ton 3/4 drop SS350 look alike

Thanks. How heavy are the full size trucks?