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Look up Stoichiometry (<--I think thats how you spell it). I think this will answer your question. Basically, for a given displacement plus the force needed to turn the motor, plus the force needed to turn the trans, rear axle and push the hole rig, you cannot get a 100 mile per gallon engine based on stoichiometry science for a 4000+ lb rig.
Basically, if anyone tells you there is a magic carbuerator that makes your 302 get 100 miles per gallon and have all the power you need to pull your truck and trailer they are full of crap or don't know what they are talking about.
Im sure you can get 100+ miles to the gallon if you had a small displacment motor on a rig that was very light. But not for your truck.
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