Actually, fuel injection has very little impact on fuel economy by itself. Fuel economy is mostly determined by combustion chamber shape, valve timing, gearing, ignition timing, aerodynamics, friction, etc. Carbs can be setup to give you good power, good drivability, good fuel economy or good emissions, but not all at once. With fuel injection and the miracle of closed loop feed back control, it is possible to get it all. Of course, the engine/vehicle designers still have to decide what kind of trade offs between performance and economy they want to make. On most bikes they have obviously gone heavily toward performance and given fuel economy very little emphasis. Motorcycles have been able to get away with carbs until very recently because their emissions requirements have been much more lenient.
I have done a lot of testing of injector sprays and they make virtually no difference in efficiency (fuel economy) in typical port injection installations. They make a bigger difference in emissions. Smaller droplet sizes are not necessary in port injected engines at operating temperatures to get complete combustion. There is enough time and energy available to get the fuel completely mixed and vaporized during the inlet and compression strokes. Smaller droplets are helpful during cold start conditions in port injected engines. I have tested ultrasonic injectors and heated injectors. There have been a lot of fancy SAE papers written about them but the supposed benefits of their "better" atomization are very hard to find. In fact, I have seen a lot of engines where a more dense, narrow spray can improve emissions and performance because it reduces wall wetting in the intake tract which is a very bad thing.
Direct injected engines must have much smaller droplet sizes because there is much less time and motion available between injection and ignition. The fuel economy benefits of direct injection are not due to the smaller droplet sizes but to several factors depending on the schemes the engine designers have chosen to use particularly lean burn stratified charge. It is pretty hard to get direct injected gasoline engines, especially lean burn ones, to meet EPA emissions requirements. It has taken over 10 years for the technology to move from the Japanese domestic market to Europe and finally just recently to the US. That is almost entirely due to differences in emissions requirements and the general indifference toward fuel economy in the US market until gas prices shot up 2 years ago.
