[j12piprius]

EonC ~ coasting in neutral ~ engine on
20 mph ~ 62 mpg (7 stop signs in 2.7 miles or else would be higher)
55 mph ~ 60 mpg (one stop sign in 125 miles)

EoffC ~ coasting in neutral ~ engine off ~ no throttle
20 mph ~ 82 mpg ~ 33% increase
55 mph ~ 54 mpg ~ 10% decrease

Estimated aerodynamic break point ~ where both will be the same ~ 40 mph

Being conservative, estimating 250 pulses in 125 miles, and EoffC used only 40% of the time, that's 100 pulses, times .05 volts would be 5 volts of drain without the alternator charging the battery. I'm not sure exactly how alternators work, but such increased drag and drain seems likely to cause the alternator to run more.

Rpms are perhaps 900 rpm with the engine on in neutral, and 0 with it off.