Given the assumptions that a modern FI car with a TB has it closed during most "no throttle" deceleration, no fuel is being injected during this time, and most gasoline engines approach maximum efficiency at a reasonable rpm range depending on the gearing, and at ~2/3-3/4 throttle...
I was wondering if it would be more fuel efficient to cruise at some speed with the engine operating at half, or more than half of it's maximum efficiency. Or, provided the driver was able to accelerate appropriately, would periodically accelerating in the maximum BSFC area, then coasting in gear, with the same average speed, result in the same, better, or worse mileage?
The situation is a little worse than you imagine; at closed throttle over-run your efi car is injecting fuel. It`s maint aining the 14.7 A.F.ratio with it`s O2 sensor and closed loop adjustments; this is to keep the CAT alive. The rationale for mpg is that you spend very little time in this mode. I build aftermarket efi controllers that do cut off fuel on over-run. These are non-smog applications without CATS. The difference in MPG is only noticed in special cases; such as here in Hawaii, where if you`re not going uphill, you`re going downhill. A recent conversion from stock on a 1979 2L efi VW van changed the butterfly valve from 20 to 21.5 MPG. There`s one and only one correct engineering answer to the closed/open engine braking question; the open throttle produces more braking effect. The reference to the truck JACOBS braking system is false. The Jake brake holds the exhaust valves open continuously in order to increase "THROTTLE LOSSES", the correct engineering term. It was at one time appllied to gasolene truck engines, and is not linked to the Diesel engine functionly, as is commonly believed.

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