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In 1970 I read about an Opel Kadett that was hypermiled with pulse and glide from 15 MPH to 45, then engine off coasting. It was in a contest in Popular Science of Popular Mechanics. The rules required an average speed of 26 MPH.
The Wagon was more aero than a sedan. They put radial tires on the car (allowed by the rules), and disconnected the vacuum operated second barrel of the carburetor). A piece of wood under the gas pedal so it would always go the ideal distance.
124 MPG.
Now if you consider the car got about 32-35 MPG at a constant 25 MPH, you can see that there is a benefit above and beyond increasing the engines efficiency.
Most IC gasoline engines average about 18% efficiency, but they can peak at 30-36% when operated at their best BSFC. Diesels also have a sweet spot but it is not as dramatic as a throttled gasoline engine, becasue the diesel never has manifold vacuum to reduce the effective in cylinder compression whe ncombustion occurs.
That being said Mercedes went from a butterfly throttle control to no butterfly in 1982. Proir to 82 they had manifold vacuum, like a gasoline engine. After 1982 the throttle control was only the injection pump.
The net gain in mileage from that change alone was 7%. I worked for a Benz dealer when the change occured in 82. They added a vacuum pump that ran off the timing chain. The throttled engines would not shut off if there was a vacuum leak in the central locking system, until you opened the hood and pushed the fuel shutoff lever on the injection pump. Not something most customers liked to mess with on a $40 k car in 1982.
Pulse and glide will improve you mileage, especially if you glide with engine off.
It takes a certain amount of HP to just spin the engine with no load.
Now a lot of diesel vehicles are underpowered compared to most passenger cars. Plenty of torque at low engine speeds but not a lot of HP for acceleration blasts.
Combine that with transmission gearing that allows lower RPM at highway speeds and the P&G window for better mileage closes considerably.
You may find it effective up to a certain speed but after than the benefits will become insignificant.
Another factor is the aerodynamics of your vehicle. Very high CDs will cost you a lot of your energy in wind resistance, expecially in a large frontal area high CD truck.
Bottom line, I would guess the P&G will probably help up to about 45 MPH. After that the aero drag will probably kill any benefit especially at peak speeds of 60 MPH and above.
regards
Mech
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