Friday, February 15, 2008

Miles Per Dollar

Have you ever encountered an idea so blindingly simple and obvious that you slapped your forehead in frustration that it didn't occur to you first? I had one of those moments the other day, reading an article that popped up on my personalized MSN portal, concerning fuel economy comparisons. The link appears broken, but the gist of author's argument was that if we focused on how many miles our vehicles travel on a dollar's worth of fuel, rather than per gallon, we might make fewer unnecessary trips and choose more efficient vehicles to start with. I agree with that logic, though from my perspective "mp$" could be even more useful as we enter a world in which the gallons we're using aren't directly comparable, and as electricity enters the transportation mainstream, resisting easy conversion to gallons without heroic assumptions and creating potentially over-optimistic assessments of the overall efficiency of plug-in hybrid cars.

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.


Anonymous said...

The best miles per dollar would be an electric car AND charging it with solar. Many states give great subsidies (due to our loving government who just spends other people's money). Take advantage. In NY, I spend $180 a month in electric for my house. I am getting solar which after all rebates and tax credits, the $50,000 solar system, is only costing $15000. If you finance the $12000 at today's low rates, you are paying approximate $220 a month for 60 months (5 years). This system will actually produce more power than I need, and will turn back the electric meter. With the extra power, i will power an electric car, and change my gas water heat to electric water. So that will eliminate my heating also. Take advantage of the government. Everyone else is getting bailed should too. Pay $0 per mile (minus maintenance of course..and as long as you dont drive more than your electric car can go). After 5 years I will be paying $0 for car energy, and $0 for home energy, and also profiting small amount of $ from feeding electric back to grid.

Geoffrey Styles said...

EVs provide low operating costs, partly offsetting their much higher up-front costs. However, combining the two most expensive energy technologies available--solar power and EVs--is a recipe for depressing, rather than actual boosting miles per dollar. The figures you claim are, of course, the result of getting someone else to pay these extra costs for you. I shouldn't have to remind you that when you "take advantage of the government", you are actually taking advantage of your fellow taxpayers, as well as the utility ratepayers subsidizing your net-metering arrangement. Your attitude towards those of us who are underwriting your solar array ought to be one of gratidude rather than braggadocio, especially when taking full advantage of these tax breaks requires an income high enough that you could probably have afforded these items without our help.