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Overall, P&G is just about how much energy out of the engine we can get compared to the amount of fuel we use. If we were fortunate enough to have ten speed transmissions like big trucks have, we proably wouldn't have to P&G much, but unfortunately we don't, and because of this we sometimes can't keep the engine operating at a speed/load where fuel consumption is minimized.
Transmission losses, at best, drop by a few percent at high torque, so there isn't a huge incentive to P&G from that end, and this can be offset by a richer mixture during acceleration, especially w/ OBDI and earlier vehicles. So really, it comes down to how far off of the minimum BSFC/maximum engine efficiency someone is during whatever driving situation. If they can move along at 35mph, or 55mph, or whatever, and the load is high enough, then P&G probably isn't worth it. If they're in an unfavorable portion of the BSFC map, then P&G may be able to help out w/ mileage.
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