As fuel diversity increases, the utility of measuring vehicle energy efficiency in terms of miles per gallon (mpg) diminishes, unfortunately coinciding with a much greater emphasis on mpg thanks to last year's Energy Bill that raised the required new car fleet standard to 35 mpg. This is more than a technicality, when you consider that carmakers get to count "flexible fuel vehicles" (FFVs) that can run on E-85 or gasoline as though they achieved higher mileage on ethanol, rather than about a quarter less. The Energy Bill, which included provisions strongly promoting E-85 and FFVs, at least limited the contribution of this factor to 1.2 mpg of a carmaker's average through 2014, phasing out to zero in 2020.
I could not find any cars that were available in all possible energy permutations, but the 2008 Chevrolet Tahoe large SUV came close. It's available in gasoline, FFV and hybrid versions. With retail gasoline averaging $2.96/gal. this week and E-85 at $2.48/gal. (both varying widely by state,) and using the EPA's fuel economy estimates for this vehicle on both fuels, the "mp$" comparison is interesting:
- Tahoe V8 on gasoline: 5.4 mp$
- Tahoe V8 on E-85: 4.8 mp$
- Tahoe Hybrid on gasoline: 7.1 mp$
While I'm sure there are locations where E-85 would have an advantage over regular gasoline, that requires it to be priced in a way that fully reflects its 25% lower energy content.
The next comparison is between gasoline and diesel, which has been significantly more expensive than gasoline this winter. There are a few manufacturers with comparable cars available in both fuels, including Mercedes and Volkswagen. Since I couldn't find 2008 diesel results for VW, I picked the former's E-series sedan to compare. Since the gasoline E350 requires premium fuel, I added $0.25/gal. to the US average price.
- E350 6-cyl. on premium gasoline: 5.9 mp$
- E320 Bluetec turbodiesel: 7.9 mp$
Finally, let's compare a Prius-style hybrid with the likely result for a plug-in hybrid, such as the Chevrolet Volt. In the absence of actual efficiency data for the Volt, I assume it would be comparable to the Prius on gasoline. Electric efficiency should be around 4 miles per kilowatt-hour. The average residential electricity price last year was 10.7 cents/kWh, with some markets considerably above that and others offering time-of-day pricing that would allow for overnight recharging at a lower price, so the following is a rough estimate:
- Prius or Volt on regular gasoline: 15.5 mp$
- Volt on residential electricity: 37.4 mp$
- Volt in 50/50 driving mix: 22.0 mp$ (very impressive, but not quite the 100 mpg equivalent often touted)
Miles per dollar has much to recommend it, particularly for its simplicity and alignment with the priority consumers put on value. However, it also has two key disadvantages. Unlike mpg, it changes every time fuel prices do, so any comparisons based on mp$ are only snapshots at a point in time. Nor does it address the emissions associated with that dollar's worth of energy, though mpg doesn't do that, either. A carbon tax or cap-and-trade system would help align fuel prices with their environmental consequences and make the resulting mp$ comparisons reflect both price and emissions. In that case, mp$ would be a significant improvement over mpg, particularly in helping consumers cut through an increasingly complex set of different fuel and power-train options. And while I don't expect Congress to rewrite the new CAFE standard in mp$ terms, or carmakers to embrace a metric that calls some of their marketing into question, how hard would it be for consumer-oriented car websites to display mp$ alongside mpg? More information might just lead to better decisions.