1-2 mile hit in range for highway speeds is what I have seen, I went for about 9 months without, then put it back on. The variation caused by winds and temperature is far greater than what the spoiler does. City driving would make no difference as the drag is not a significant part of the range

 

The front air dam is for high speed stability....to keep air from lifting the front end.
Any dam offers more wind resistance and therefore more drag. (lower mileage)
The Volt presents a lot of flat area behind the air dam.

 

Not necessarily. It directs airflow away from the rough underside that usually would increase drag. I think GM didn't spend their wind tunnel time for laughs. It's part of the reason the production Volt doesn't look exactly like the car show prototype.

 

You'd need access to GM's wind tunnel tests I think. Or someone with a test track and a lot of time on their hands.

 

Or we could meet at 11 pm on a night when the temperature was going to stay fairly stable and there was no wind and we could drive 20 miles west and then 20 miles east with long air dams on our Volts and then take them off and replace them with the short air dams while our cars are charging and repeat the test at the same speed!


Or we could discuss it here.

I think discussing it here makes a lot of sense.

;-)

Also, back in 2011 Adorandax posted about his experience both before and after going from long to short air dam, and it was close to a wash. Guyflyguy noted that he had been told that it would take off 0.6 mpg at "highway speed" but Don C noted that he had heard that it took 1.3 miles off the EPA test cycle. Oranges to apples but they appear to be slightly different.
http://gm-volt.com/forum/showthread.php?8594-Tale-of-Two-Air-Dams

 

We took our airdam off and I don't think it's made any difference. However, most of our miles are below 45 mph. Living on an island can be completely frustrating sometimes. Even when the posted limit is 55, most locals drive at 50 or less. Also, no freeways.

 

http://gm-volt.com/forum/showthread.php?8594-Tale-of-Two-Air-Dams[/url]

As far as I know the EPA tests are done on a dyno, not an actual road course. The aerodynamic effect would have to be modeled and factored in wouldn't it?

 

Ah, the penalty you pay for living in paradise!