The trick isn't to change the delivery rate of the fuel injector to increase economy - the trick is to improve the dispersal pattern of the fuel injector. That's why a lot of times you'll see a slight gain in economy after swapping out for Ford "four-hole" injectors - the finer atomisation of the fuel allows for more thorough combustion, using less fuel.
Note that the terms "economy" and "efficiency" are different - "efficiency" refers to the consumption of a given amount of fuel per unit of engine power produced. "Economy" refers to reducing absolute fuel consumption (maintaining the same basic engine configuration) per unit time.
Economy and efficiency, however, can be both increased (in limited amounts) by the same mechanism - in this case, improving atomisation of liquid gasoline, which will serve to increase the surface area available upon which to begin combuston, or by very nearly vapourising it, thus reducing the heat input required to begin combustion in the first place.
By more finely dividing the liquid gasoline ("improving atomisation,") more efficient combustion can be had, which improves fuel efficiency (somewhat less fuel needed to release the same heat output at peak burn times.) This can also improve economy - since you need less fuel to do the same work, you'll use less absolute fuel.
Granted, the best way to increase economy is to reduce the displacement of the engine (didn't Cadillac try something like that with the "4-6-8" system in Northstar?) but that is taking economy as an absolute value, and not as a relative (in this case, "relative" would refer to various engines, using the same basic setup, using differing quantities of fuel to do the same amount of net work.)
Also, the gains you're likely to note in economy from doing something like this would be nominal at best - you're likely to realise savings when you take fuel consumption over periods of 40,000-50,000 miles, and not by the tankful. Most fuel-injected engines (particularly port fuel-injected engines, like ours,) are already tuned to be fairly efficient from the factory. Improving them would require an engine management system allowing for user input and reprogramming of various values (elsewise, the ECU will try to "correct" your modifications through its own adjustments, since it has no idea what you're trying to do. Bear in mind "production" vehicles use "best-fit" programming, aren't tuned to allow for individual variation, and the programming has to make allowances for manufacturing tolerances - therefore, while it "fits" the engine it's running, it's not a "best fit" - and nearly all production vehicles could benefit from some individualised tuning.)
So, the single best modification for economy is what they won't allow us to do - get under OBD and reset a few values to fit that particular engine and that particular management system. Why? Who knows - considering efficiency and economy are not mutually exclusive goals ("efficiency" not only refers to the power made per unit of fuel consumed, but also to the total use of per unit fuel - meaning greater total combustion, and thus lower aggregate harmful emissions...) but EPA and CalEPA seem to continue to suffer from the delusion that none of us know anything about how engines or engine management systems work, and they do (yeah, right) - CalEPA in particular! Even though, for intance, my vehicles are all "pre-OBD," modifications are not allowed (never mind having the ability to prove - using their own machinery and test methods - that removal of an emissions device or two will actually reduce aggregate tailpipe emissions!)
Granted, as OBD-II is advancing, they're doing a better job of allowing more flexibility in engine controls (for manufacturing tolerances, individual variations, environmental conditions, et al,) but I'm assuming we're talking about OBD-I and pre-OBD vehicles here, no?
So, how much you can do to improve fuel economy (consumption per unit time operation) is really a variable quantity - which decreases significantly circa 1996/7, and is on a steady downward trend since. It's always been on a steady downward trend, but there was a sharp drop in the 1996/1997 timeframe, since SAE was the governing body for OBD-II (while CARB was in charge of OBD-I, initially.) Sharpish drops would also be noted at the shifts from carburettors to throttle-body injection, and from TBI to port fuel injection. One can expect a simliar drop to take place once Bosch gets their "Direct Injection" to work - similar to Diesel fuel injection (the Diesel is actually a fairly efficient powerplant in terms of fuel used per unit power generated - it's just not emissions-friendly. But that is also changing...)
