44 mpg in Mountain Mode - help me understand this...

[notaguru]

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

[saghost]

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.

Where did you get this 44? If you're reading the number directly off the energy screen in the center console, that's the average for the trip overall, discounting electricity but including the miles driven electrically. You won't see it changing with sudden changes in driving style because it's a full trip average. The easiest way to get actual mpg for a driving style or section of trip directly from the car is to reset one of the two trip odometers at the start of it. Then you'll have average MPG for the section calculated in real time...

[tboult]

The car does not have an instantaneous MPG reading.. so changing speeds has no effect on the display.

It was stuck at 44 for a while because it was the average of your MPG_CS (which may have been say 38-42) with the EV miles counted.. probably dropped a bit as you climbed the mountains at high speed as the ration of CD to CS continued to slowly drop.

In CS mode, or in MM mode, if the battery has enough juice both the engine and battery supplied power can be used for passing. If in CS mode and the SOC is way down (e.g. just about to cycle the ICE on or you've been in CS mode going up long hills) then the battery may be limited in what it can supply.

[notaguru]

Thanks --- I had thought that the mpg number was instantaneous performance. I'm sure the computer knows it (we've had less brainy cars that display that datum) and don't understand why it's not displayed somewhere.

Good to know about ICE+electric for passing. It's pretty impressive.

[tboult]

Thanks --- I had thought that the mpg number was instantaneous performance. I'm sure the computer knows it (we've had less brainy cars that display that datum) and don't understand why it's not displayed somewhere.

Good to know about ICE+electric for passing. It's pretty impressive.

You can get the instantaneous MPG via OBDII.. just about any OBDII device/software will show that field.

[saghost]

Thanks --- I had thought that the mpg number was instantaneous performance. I'm sure the computer knows it (we've had less brainy cars that display that datum) and don't understand why it's not displayed somewhere.

Good to know about ICE+electric for passing. It's pretty impressive.

Because it'd confuse people, I think. It confused Car & Driver when they used OBD data to get it:

http://www.caranddriver.com/comparis...omparison-test

If you look at the chart of mpg vs speed, you'll note that they thought the Volt gets worse mileages at lower speeds for speeds below the middle 60s. That's because they took instantaneous readings at a steady speed with the engine on - perfectly reasonable on any other car they'd come across, but missing the fact that the car was charging the battery at those speeds, and charging it faster at lower speeds (at those lower load roads, the Volt cycles the engine in steady state driving - several miles on, a mile or two off, then repeat.) (Since then, the Hyundai Sonata Hybrid and Kia Optima Hybrid have come along, both cycling the engine at freeway speeds as well.)

Without the information about how the battery SoC is changing, instantaneous mileage on the Volt is meaningless, and if a major automotive magazine couldn't figure that out, how in the world would the general public be expected to? So you'd either have to show power sent to the road and to the battery and some kind of "road instantaneous mpg," or you'd have to average it with power off time - thereby making the instantaneous an artificial average number instead. To do anything else would be to encourage misunderstanding of what in other cars is a very simple number. Have I mentioned the Volt is a somewhat complex and rather unique car?

[Bob G]

I did a trip over the mountains and back and noticed the same thing. The display was claiming over 42 MPG (as if electricity was free), when I was actually getting 33 MPG (@70 MPH; full of camping gear).

[marlow]

Yes the fact that the Volt counts the electric miles in calculating the gas milliage buggs me to, its like deceptive advertising, but it is what it is. (actual mileage may vary).

As previously said, you need to reset one of the odometers when the ICE comes on to get an accurate gas mileage reading.

And dispite the errors in the article, It did point out that the Volt gets excelent gas mileage at highway speeds with the low air drag and when the ICE starts suppling some power directly to the wheels, its like a super over drive.

On a nice day, it is easy to actualy get over 40 MPG at 70MPH even while using the AC in Comfort Mode with four adults in the Volt.

So don't be afraid to see the USA in your Chevy Volt, its great!

[saghost]

Yes the fact that the Volt counts the electric miles in calculating the gas milliage buggs me to, its like deceptive advertising, but it is what it is. (actual mileage may vary).

As previously said, you need to reset one of the odometers when the ICE comes on to get an accurate gas mileage reading.

And dispite the errors in the article, It did point out that the Volt gets excelent gas mileage at highway speeds with the low air drag and when the ICE starts suppling some power directly to the wheels, its like a super over drive.

On a nice day, it is easy to actualy get over 40 MPG at 70MPH even while using the AC in Comfort Mode with four adults in the Volt.

So don't be afraid to see the USA in your Chevy Volt, its great!

The ICE will supply power directly to the wheels at speeds as low as 35 mph - the only thing that is special about the 65-70 mph range is that the road load is higher than the engine output at WOT at minimum rpm (1400,) so the car switches to a HSD type eCVT strategy. By sticking to the (mostly 55, occasionally 65) speed limit I recently got an honest 47 mpg on the gas portion of a ~200 mile road trip (in addition to 46 EV miles.)