I have an automatic transmission car with a sport shifter. The kind for people who don't know how to use a clutch. Anyway, I like to downshift, you know, rev the engine a bit or sometimes just to slow down, but my question is, when I do this, does it ruin gas mileage? I don't press down on the gas when I do this because I am usually coasting at the time. Is it still injecting fuel into the cylinders?

Can it ruin gas milage. Yes. Is it because its injecting fuel to do the work? No. It because when you slow down by compression rather then easing to a stop you have a greater chance of making corrections and that is where you lose the milage in your car.

Thanks