[t vago]

Less fuel being injected may mean less available energy, but in the case of lean burn, it also means a larger percentage of that energy being used to do useful work.

You also have to figure out what your volumetric efficiency (VE) is at 1500 and 1700 RPMs. This is important, because lower RPMs doe not automagically mean better VE.

F'r instance, the 4.7L V8 and transmission package used in my truck originally had cams and intake that provided decent performance for a truck, though not anything special. The transmission computer that controls my transmission originally gave 4 forward gears (one overdrive).

The High Output version of this engine/transmission combo was developed for the Jeep Grand Cherokee, and this involved (among other more subtle things) changing out the cams and intake manifold for items that had better VE at lower RPMs, thus allowing the engine to gain more torque at the low end. This dovetailed rather nicely with the transmission computer reprogramming that allowed the Jeep Grand Cherokee to have 5 forward gears (2 overdrive gears) using the same exact transmission hardware.

Now, I retrofitted my engine to have these High Output cams and intake manifold, and retrofitted the newer transmission computer to my truck about 2 months after I swapped out the other parts. As a result, I could also have that second overdrive gear which allowed me to go down the highway about 300 RPMs lower than before, and I saw a gain of about 1 MPH with the new transmission computer, as opposed to the old one.

Other Dakota owners with the 4.7L/transmission combo and the programming for 4 forward gears, on another website I frequent, only changed the transmission computer by itself, while leaving their intake manifolds and cams alone. They reported different results, in that they saw their RPMs drop by about the same amount as what I saw, but also saw losses of fuel efficiency as a result of the revised transmission computer. Their engines did not have the modifications that improved VE at lower RPMs, thus their engines had to work harder than they did before, just to suck in air-fuel mixture at those lower RPMs. As a result, their FE dropped even though they added a second overdrive gear.