<font color="red"> then the second part of your assumption is wrong. </font>
Too strong a statement, of course, but I was writing it late at night (for me /forums/images/graemlins/crazy.gif)
If 30 hp wasn't enough to hold the mower rpm and turn the wheels, then both will slow. As they slow, the gasoline engine will fall farther away from its power peak than the diesel, and thus slow more. So, what you described may be exactly what happens. After slowing, the diesel will deliver more power than the gas engine -- but no longer 30 for either..
I think the only point I really am trying to make is that the potential work of the machine is determined by available power, which is torque times speed, not torque alone. 30 horsepower machines will do equal work, provided they are geared to deliver the whole 30 hp. If not at the power peak, 25 hp will beat 19 every time.
It's not the temperature, it's the humidity. /forums/images/graemlins/grin.gif