Yesterday an associate and I drove east then west across the same mountains (sea level to +4000' to below sea level, then return), for a total of 241 miles. I drove the eastbound trip at an average speed of about 75mph, and my co-driver did about 85-90 most of the way back. The car was in Mountain Mode both directions.
We had about 100# of gear in the back. Outbound, morning air temperature was about 75degF. On the afternoon return trip it was about 109degF for the first 40 miles, then as we climbed into the mountains it dropped to the mid 70s.
The car used a full charge plus 6.8 gallons of gas. Assuming about 37 miles on the charge outbound in the morning, that's 30mpg on gasoline overall. Not bad, considering we were driving like it was a race through the Alps.
This is interesting. After the first 80-90 miles outbound (past the mountains), I thought to check the Energy screen. This was in the flats but still in Mountain Mode. MPG showed at a consistent 44mpg or so regardless of speed. I slowed to 50, accelerated to 95 - no change. I assume the engine runs at a fairly constant rate, an optimized rpm for economical electrical power generation. Therefore, fuel flow should be fairly constant, right? If so, then with speed variation, shouldn't mpg vary as well? Curious.
Another thing - passing performance. There were a few times when we needed to pass and there wasn't a lot of room. In Mountain Mode, we got the feeling that both the ICE and the motors contributed, and even at 60-70mph the acceleration was respectable, even impressive.
Overall it was a great trip, especially the return; terrific weather, wonderful Bose audio blasting away, and two geriatric cases singing along with Sinatra and Como (thanks, Sirius CH 71) as we roared along, passing everything in sight.
But the mileage... the economy... is a puzzlement. The cost of the trip:
Electricity: actual cost is zero due to my plan*, but call it $1.60
Gas: 6.8 x 4.00 = $27.20
Total = $28.80
Per mile = $0.119
That ignores the cost of tires... we left rubber on the pavement of a few curves marked CAUTION 40MPH as we careened around them at twice that speed.
*ownership of a plug-in car reduces my overall house rate, so I really pay nothing to drive the car. Charging it daily, my monthly bill is less than it was a year ago without the car, but at the higher rate. Considering that benefit of owning the Volt, the cost of this trip was just gas:
Gas = $27.20
Per mile = $0.112