EOC ~ only useful at low speeds
 

The mpg using EoffC on my trips has been much less than with leaving the engine on when coasting. For example, two previous 125 trips averaged (1) 54.0 mpg using EoffC all the way, then (2) 59.9 mpg using EonC, averaging about 55 mph. Care has been taken to shift at lower speeds, and to not lose momentum with shifting, so the continued poor EoffC result has been puzzling. My kill switch is connected to the fuel injectors.

Recently I've noticed that switching the injectors back on (with the engine off), drops .05 volts from the battery, thus 20 switches drops the battery from 12.8 to 11.8 volts. Perhaps this drop results in the alternator working harder while driving, uses more fuel, and causes the lower mpg.

My hypothesis is that EoffC is only useful below a certain break even point, depending on the aerodynamics of the car. For example, my car gets 30% better mileage using EoffC when averaging only 20 mph, but 10% worse mileage averaging 55 mph, with an estimated break even point of 40 mph. Better aerodynamics would raise the break even point higher.

I don't know about the electrical bits there, but at higher speeds your throttle is more open already, so less return from p&g.

That's a 10% difference. What mileage obtains with no p&g? Does the voltage not rise again with the engine back on?

Same speed. Same aerodynamics. Small sample size. Maybe you'd do well to have the mods move this to General Efficiency.

Looking at the glide distance, aero drag is stronger at higher speeds so you'll lose speed faster at higher speeds, which cuts its utility. Looking at fuel consumption while gliding, you'll have more injector pulses per foot travelled at lower speeds, so the benefit of engine off is magnified there.

But I wouldn't say it's only useful at lower speeds, just that at higher speeds you need a longer glide for it to make sense, generally with some sort of a downhill to help extend the glide while minimizing speed loss. It doesn't take much of a downhill to help- one of my morning EOCs is on a stretch of the highway that I hadn't realized was an incline until I started hypermiling.

But yeah, higher speed reduces your ability to save fuel.

Isn't that the truth?? Wow, I was clueless to some little gold mines. AS to the OP, Just like he said: higher speeds will largely negate anything you can do, besides slowing down.

How many RPM's at 55MPH?

aerodynamic break point
 

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.

That is where you lose me, are you bumpstarting? Is your ecu going into a weird mode (would a cam or crank signal interupt prevent it)?

It shoud always use less power when off.

RPM's in 5th gear at 55MPH?

Yes, bumpstarting.

The battery use is much higher.

Rpms are not relevant here, as the pulses are the same, but somewhere around 2500 rpm.