hazmet:
<font color="blue">Unless you are farming full time, fuel costs are small potatoes compared to the other costs of ownership (such as, the hidden - gotta get a new implement every 6 months cost </font>
You got that right. How do you hold implement purchases down to one every 6 month?
<font color="blue">For example, to mow 2 acres takes a certain amount of energy or HP-hours. Lets say it is 30 HP-hours. . . . </font>
Your explanation may be technically correct, but it "real life" may not work out that way. If each tractor is working only at partial RPM, running the same width implement there will be little difference in the time to completion, especially if the mowing is "light duty". Thus there would be a slight difference in fuel useage for the different size engines since a greater % of the fuel consumed would be to run the engine and less for external work performed. But, as you say, it's trivial.
Went to pull fuel useage from my log and must have forgotten to write down some purchases. With 130 hours to date, useage seems to be running less than .5 GPH (on a TC40). In any event, as you say, useage on these is very, very low. Whatever he gets, fuel useage will be a drop in the bucket (or in the fuel tank!) compared to other costs of ownership.
JEH