[DucFanDan]

Quick update: I've tested 25, 30, and 35. It seems to like 35. Haven't tried 40 yet.

Down at 25, it downshifts and revs too much on the grades... just gets bogged down and slurps down fuel at the rate of about 8mpg.

At 30, it downshifts only on the steeper grades, and drops down to 9.0-9.5 mpg according to the readout.

At 35, it will hold gear on all but one steep grade, and it only dips below 10mpg briefly on the steepest grades.

Still need to try 40.

Bear in mind, when I give a speed, that's just my "target speed"... I let it rise up to 5 mph above when coasting downhill, and let it drop 3-5 mph when climbing.

So... thus far it appears to be most efficient on gravel in the same speed range it is most efficient on pavement. There may be a small difference... maybe peak is at 35-40 on gravel where it is 40-45 on pavement. Not sure yet, but they're shaping up to be close.