Generally when you increase horsepower and torque on a diesel engine it doesn't have to work as hard to maintain speeds as it had to before. So in general to answer your question fuel mileage will almost always go up when any power is added via chips, propane, water methanol, injectors etc as long as you can keep your foot out of it. DieselSmoke.com has pretty decent stuff try them
Then why do most semi have 470hp or 500hp when you can get 600hp from all 3 big diesel engine builders.
Because horsepower doesn't move freight. Torque does. Horsepower is just a.... Ah, nevermind.
Way back when I was driving over the road, a friend who worked at a dealership explained it to me once. He said that the engine makers don't sell "engines", they sell "power". Given that the old Cummins six cylinder in the '70's was typically rated for 290, 350 or 400 hp, the price of a truck depended partly on the power rating of the engine.
If you went to an "authorized service center", they charged extra for "turning it up", but your shadetree mechanic didn't have to, but then you got into the warranty discussions.
Greg
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