I am and always have been a "down shifter" when coming to intersections instead of using the brakes exclusively, now with my limited knowledge of engines my thoughts was that with a carborated engine that under deceleration the movement of the engine would pull more gas through the carborator. this would cause lower gas milage than if you just let the engine idle when coming to a stop.
now watching my mpg indicator while driving on my car i noticed that the mpg shoots up while decelerating whether or not i downshift. that got me thinking and i figure with fuel injection it probably simply stops 'injecting' fuel so why would it lower your mileage by downshifting.

so my question is am i right? does a carborated engine pull gas in while decelerating? does a fuel injection car not pull any excess gas into the engine? am a way off the mark?

 

Well, mpg is mainly effected by load, not rpm. when you are decelerating, there is no load, just rpm. When you are trying to pick up speed, you may use more gas at low rpms than higher rpm. Focus on load not so much the rpm.

 

Yes, you are pretty much correct; as far as i know all modern fuel injected ECUs shut off the injectors when de-celerating, specifically to save fuel. To do that they sense inputs such as throttle position, MAF, etc.

Carbs, in contrast, are dumb, and only react to the amount of air passing through them. I believe there were some attempts (early to mid 80s?) to make them smarter; solenoids were added to allow fuel shut-off, etc. But eventually they were all replaced be FI for obvious reasons...