If the recent flap over the Volt’s drivetrain has taught us anything it’s that A) GM’s internal-combustion-assisted plug-in is more complicated than we thought, and B) GM is fine with simplifying its complex reality in order to make it appear as attractive as possible. Which is just fine: they’re the ones trying to sell a $41k car, and as such they’re entitled to do what they can to make it seem worth its many shortcomings. What the automotive media needs to take away from the brou-ha-ha isn’t necessarily that GM’s hesitance to bring forward “the whole truth” is an intrinsically big deal (let’s just say this wasn’t the first time), but rather that knowledgeable writers should focus on explaining the Volt in ways that are both comprehensible and fully accurate. In this spirit, the most important question isn’t “what should we call the Volt?” but “how efficient is the Volt in the real world?”And on this point, there’s plenty of room for some truthful clarification.
But as with questions of the Volt’s taxonomic category, objectively analyzing the Volt’s efficiency requires a certain amount of semantic clarity. After all, unlike any car that has come before, the Volt operates in two discrete modes: “EV mode,” in which it drives on pure electrical energy, and “Charge Sustaining” or “CS Mode,” in which its gas “range extending” engine generates electricity and (under certain circumstances) even sends torque directly the Volt’s wheels. Unlike a Prius (or other parallel hybrids), the Volt doesn’t continuously vary between gas and electric drive, but runs on pure plug-in power until the battery reaches 30 percent capacity, and then switches to CS Mode.
Because of this drivetrain concept, it’s crucial that EV Mode range is presented separately from CS Mode MPG. After all, combined MPGs from a trip using both EV and CS modes will depend entirely upon the length of the trip. If a trip lasts (say) five miles longer than the EV range (say, 40 miles in EV mode and five miles in CS mode), and the car returns (say) 50 MPG over those last five miles, it will have burnt .1 gallons for the entire trip, resulting in an average MPG of 450 MPG. But if you charge the car’s battery once (giving another hypothetical 40 miles of EV range) and then drive 450 miles (410 of which burn gas at the hypothetical rate of 50 MPG), you’d be lucky to get 55 miles per gallon for the trip. In short, how often you recharge the Volt’s batteries is the single defining factor in determining an overall MPG rating for the Volt.
And this reality is already leading to confusion. Over at Motor Trend, former TTAC writer Jonny Liebermann claims to have wrung 127 MPG out of his Volt tester, gushing
Broken down, over the course of 299 miles on Los Angeles highways, byways and freeways, the Volt burned 2.36 gallons of gasoline (fine, 2.359 gallons — we rounded up). Most other cars use up a tank of gas going 299 miles. The Volt, to reiterate, used 2.36 gallons over 299 miles. That’s freaking amazing!
Unfortunately, Lieberman prevents us from concurring with his breathless assessment by failing to include an accurate log of the trips he took to achieve that number. Working backwards through his less-than-scientifically-presented data, we reckon Lieberman got about 52.6 MPG in CS mode, with at least one battery recharge somewhere in the middle. That’s better than the consensus forming around the Volt’s CS-Mode efficiency (somewhere around 35 MPG according to Popular Mechanics and Car & Driver), but it’s still nowhere close to supporting 127 MPG as a consistently-achievable CS Mode rating (it’s also a number that GM would likely not stand by if pressed).Unsurprisingly, this hasn’t prevented GM’s social media and PR team from sending copious numbers of detractors to Lieberman’s piece by way of “educating” them.
Of course, Lieberman’s analysis is not without value: it shows that the Volt can achieve 127 MPG in “normal” driving. But with an extra charge-up somewhere in his poorly-defined test, he could easily have achieved twice that number (think the infamous “230 MPG“). Without an actual trip log showing how much gas and electricity were used on each leg of his test, however, Lieberman’s 127 MPG number means nothing. By using it as proof that the Volt is indeed a “game changer” GM’s PR team is simply proving the extent to which they’re willing to exploit confusion (even among the august ranks of buff book contributors) in order to hype their car.
At the end of the day, the Volt’s efficiency will most accurately be represented to consumers by outlets that test EV range and CS mode range independently and transparently. Actual EV mode is important, because it will give consumers a real-world sense of how far they can go without using any gas (i.e. whether their commute is short enough to use the Volt regularly in EV mode alone). CS mode MPG is also important because it tells consumers what they can expect when the Volt’s EV range is expended. Any attempt to fuse the two into a number that can be compared to the MPG of a Prius or Escalade must be clearly qualified by a transparent accounting of the trips used to get that rating.
Thus far, a consensus on these points is building:
Popular Mechanics calculates a 33 mile average EV mode range, and between 32 and 36 MPG in CS mode.
C&D claims between 26 and “the upper 30s” for EV range in miles, and 35 MPG for CS mode.
We’ll keep a close eye on efficiency test results for the Volt, and report them here at TTAC in terms of EV range and CS mode efficiency. Anything else would be less than the whole truth.