What do you think affects gas mileage more?

Engine technology helps a little. According to fueleconomy.gov, a '98 C1500 Suburban picks up 1 mpg over a '91 R1500 Suburban. I wouldn't be surprised to see you pick up 1 mpg if you installed a newer 350. Of course, what is it about the newer 350 that gives it the extra mpg: the better flowing heads, the engine management system, or ???

I expect it would take years to recover the cost, unless you had some other reason for replacing the engine, such as the existing engine needs rebuilt anyway.

MrShorty said:

Engine technology helps a little. According to fueleconomy.gov, a '98 C1500 Suburban picks up 1 mpg over a '91 R1500 Suburban. I wouldn't be surprised to see you pick up 1 mpg if you installed a newer 350. Of course, what is it about the newer 350 that gives it the extra mpg: the better flowing heads, the engine management system, or ???

I expect it would take years to recover the cost, unless you had some other reason for replacing the engine, such as the existing engine needs rebuilt anyway.

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Yeah, it wouldn't be to save just gas. The engine seems pretty strong. In the event it goes, I figure instead of a rebuild, it would be nice to replace, to get more power, and maybe some better gas mileage to boot.

Given the weight of the Burb I would say the energy needed to accelerate it from standstill/low speed to a higher constant speed is the BIG fuel consumer. Accordingly, the biggest impact on fuel consumption is the way you drive. If you manage to keep your going smooth and without pumping the accelerator, you can avoid some of the biggest fuel thieves. Second I would say is the not to good aerodynamics at higher speeds (75 mph and over). This however is a pure guess.

Everything else would have marginal effect.

(A flexifuel might have some good effects on the environment, though, if that's a consideration)