Fuel Saving Strategies

[chevy]

Is it better to use a smaller tractor running flat out or a larger one at lower RPMs? Also, with MFWD, I've always thought that if you are pulling anything on a surface besides pavement, it can actually save fuel by reducing wheel slippage - am I off here? I turn it off when traveling fast on roads (although on the deeres it engages as soon as you touch the brakes), but otherwise leave it on pretty much all the time for tillage, digging, mowing, etc. What sort of fuel additive do you use and does it work?

I learned several years ago when mowing with a mid mount mower, that if I ran the tractor at few hundred RPM below the rated PTO RPM I used a lot less fuel. I have never noticed any difference in the cut.
For other tasks, I run the tractors at RPMs just high enough to get the task completed.