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· Super Moderator
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Engine technology helps a little. According to, 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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