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
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
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.
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