1900-1975
When you downshift with the throttle closed, you create an high emission spike as a little extra fuel is pulled thru the idle circuit of the carb, oil is yanked up through the rings, and sucked down the valve guides. there is so much of this stuff that if your tailpipe is loose, it will explode in the tailpips as cool O2 is intoduced through the leak!
1975-1988
To prevent this, the Industries geniusses have brought us distributors with vacuum retard as well as advance, air injection, throttle return delay dampeners, and even on my 84 Chevy 4x4, a device which pulls the gas pedal from under you foot if vacuum is too high!
1988-????
Well, nowadays all that is a thing of the past, most of the better EFI systems stop foring the injectors untill RPM have dropped to 2 or 3 thousand. The IAC motor lets some extra air by so oil is not pulled into the combustion chamber.
A good hybrid car will at this point be using its motor to recharge the batteries too.
Neato!


"much more significant is the way you accelerate....trust me one of my bricks has an mpg right now guage and it can get frightening (5 mpg! without trying!)"
TRue, it is depressing to watch, but there is a great deal of evidence that brisk acelleration to high gear, followed by coasting as much as possible is the most fuel efficient way to run a conventional car.

Indeed, last summer I was able to average 15 MPG over 100 miles of rolling hills in Nevada (4000ft?) in a 12,000lb+ Mercdes D 309 bus running a 350CI 4bbl carbureted motor, non overdrive automatic, and 4.56 gears. I was averaging 35 or 40.
On my way home with much reduced load(8000lb?), at night so as not to have to fight the overheating that made me 'burn and coast' in the first place, I got on it with the semis and drove downhill to CA. I got 9 MPG on the return drive and never went faster than 70.