Increases in performance or fuel mileage are possible, but the two are usually impossible to achieve at the same time. Short duration high lift cams are known as "torquers" and they produce gains in low-end torque that would be useful for trailer towing and general-purpose load hauling. Long duration cams with lots of overlap (intake valve opening while the exhaust is still closing) produce a rough idle and poor low end torque, but they increase horsepower at high RPM. Most hotrod cams are the long duration type, with a somewhat higher lift and a faster rate of lift. Bad news for trailer towing or load pulling unless you keep the engine wound up in the lower gears a lot.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
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