I agree with when you run the tractor engine slower under load it takes less fuel.  I bought a new to me 8200 and had it on my old 21 ft disc.  I checked the fuel used per field, I knew the acres, in a lower gear with a high eng. speed  vs. another field of know size with the tractor up 2 or 3 gears and much lower eng. speed.  The lower speed eng. took much less fuel per acre.  My understanding of eng. chips means that they will save you fuel by allowing you to do the same work at a slower rpm.   

I was once told that it takes so much fuel to do a job with an eng. be it ran fast or slow.  Not true.