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Posts Tagged ‘mpg’

GPM Sheds Light On Why Hybrid Cars Are Tough To Justify On Fuel Savings Alone

by August 23, 2011

Hybrid car parking only

Photo credit, Chris Bloke on Flickr

Have you ever considered buying a hybrid car and gone through the calculations to compare the fuel cost savings to the higher price over comparable non-hybrid models? Chances are good that the math is not on the side of purchasing a hybrid, as pointed out in this USA Today opinion piece questioning the recently-announced, aggressive fuel efficiency standards. Most likely, it is our choice of metrics for evaluating fuel efficiency that help to make such calculations turn out to be less intuitive than we might expect, as discussed in this previous post on fuel economy metrics.

Hybrid cars are those that combine a large battery with an engine that generally runs on gasoline, although diesel hybrids are possible. In city driving with lots of starts and stops, the energy normally lost through braking is used to incrementally charge the battery—through a technology known as regenerative braking. If you’re looking for a 2-minute diversion, check out this video below featuring Maxwell von Stein, whose video was featured on Science Friday last week. He created a bike that uses a flywheel to store energy from braking to provide a boost literally down-the-road. He sees this as a great test case for creating a new type of hybrid car…

Because we need to brake less on highways, hybrids lose much of their advantage during highway driving. You can check out the EPA city/highway ratings for hybrids and compare them to non-hybrids side-by-side at fueleconomy.gov.

Following up on the previous post on MPG v. GPM (that is, miles per gallon v. gallons per mile), here are some quick calculations showing the fuel savings for two models from Toyota (Camry) and Ford (Fusion) that are available as hybrids. As in the previous post, the orange bar shows the fuel savings in gallons for 5000 miles of city driving.

thought_experiment_hybrid

The Fusion Hybrid driver could anticipate saving about $1000 on fuel over three years if she/he drove 5000 city miles, and the Camry Hybrid driver could expect about $700 in savings for three years of 5000 city miles—these calculations assume a gas price of $3.50 per gallon.

Grabbing some rough prices from Motor Trend’s site, it would cost about $6800 more for a Fusion Hybrid and about $4400 for a Camry Hybrid (these comparisons are very rough and are based on models that are one step up from the base offering for the non-hybrids; they also do not include any applicable rebates, deals, etc.).

Baring any government subsidy to help offset the purchase price, we immediately see the quandary facing a potential purchaser of a hybrid car: it is very tough to imagine recovering the extra purchase price through fuel cost savings. Plus, as mentioned above, if the hybrid was to be used for a good bit of highway driving, the potential fuel savings would diminish considerably.

I argued in the aforementioned post that GPM is a better way to get an intuitive feel for fuel savings. Interestingly the fueleconomy.gov site offers vehicle stats in GPM rather than MPG. I have to wonder if many people click on that option. Here are the side-by-side comparisons of the hybrid and non-hybrid Fusion and Camry models. First, using MPG as the comparison point, then with gallons per hundred miles (GPM). The cost savings tables at the bottom provide the same type of thought experiment as described above, though it is a bit more complex blending highway and city mileage.

fueleconomy_gov_mpg fueleconomy_gov_gpm

It would be a little like getting a nation to think metric, but one gets a sense that the second comparison provides information that is more instructive about cost savings from fuel usage.

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Using Miles Per Gallon Ratings To Reduce A Nation’s Oil Consumption Is An Indirect Approach

by August 2, 2011

ap_video

AP Video Coverage of President Obama's Announcement

The new agreement between President Obama and automakers, nearly doubling fuel efficiency for automobiles by 2025, will have a direct impact on oil consumption by the U.S. automobile fleet. Yet, if this aggressive goal is fueled by a desire to maximize reductions in oil consumption, then it would be more useful to measure our performance with a direct indicator of oil consumption: gallons per mile (GPM), the inverse of our standard metric, miles per gallon (MPG). (more…)

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