October 21, 2011

What is greener: drive your old gas guzzler it till it dies, or trade it for a new gas sipper?

CelloMom goes digging into the lifecycle analysis for automobiles, and makes some astonishing discoveries.
From the green perspective: trade it in!
From the greenback perspective: keep it till it falls apart.

Those of us who worry about carbon emissions from our tailpipes, often wonder whether it would make sense to buy a new car with better fuel efficiency. After all, that new car takes energy and raw materials to build, and carbon dioxide is emitted during its sourcing and manufacture.

This is where it makes sense to do a lifecycle energy analysis, in which one considers the total energy required to produce something, in this case passenger cars, and to run it during its lifetime. There is a nice summary in a Google Answers thread, which contains many citations. MIT's Lab for Energy and the Environment (LFEE) published a report called "On the Road in 2035: Reducing Transportations' Petroleum Consumption and GHG Emissions" which contains lifecycle analyses of various types of cars; GHG stands for greenhouse gases.

In this post, CelloMom uses the numbers from the recently updated and extremely thorough research done at Argonne National Lab, culminating in a model for Greenhouse Gases, Regulated Emissions, and Energy Use in Transportation, or GREET for short. This team has considered everything, from the obvious emissions from burning the gasoline (including the well-to-pump energy cost), to the energy cost of mining and refining the lead for the car's battery, to the brake fluid and the plastic insulation around the electrical wires. Everything.

You can download the full-blown GREET model and play with it on an Excel spreadsheet. CelloMom, who gets agitated in all the wrong ways by Excel, was content to read the report on how the model was built (e.g. what assumptions went into it), which highlights some of the salient conclusions.

CelloMom is still reeling a bit from some surprises encountered in this report.


The GREET report considers the "vanilla" car weighing 3330 lbs, and reaching a total lifetime mileage of 160,000 miles. At 24.8 mpg, that requires 6452 gallons of gasoline at the pump.

Surprise: It takes about the same amount of energy to produce a hybrid vehicle and a conventional gasoline car with an internal combustion engine:

This Vehicle-cycle Energy is about 100 million BTU = 100 GJ (GigaJoules) for a car weighing 3330 lbs (use CelloMom's cheatsheet on the units for energy). The carbon emissions is about 8500 kg CO2 / car.

For gasoline engines, the energy burnt during the car's lifetime is 121 MJ/gal times the total number of gallons of gas consumed. So:
Operation Energy = 121 MJ/gal * 160,000 mi / 24.8mpg = 781 GJ
Operational CO2 = 8.80 kgCO2/gal * 160,000 mi / 24.8mpg = 56,800 kg CO2

The fuel cycle accounts for the well-to-pump process of getting the oil out of the ground, refining it and transporting it to the gas station:
Fuel-cycle energy = 781 GJ * 17 / 83 = 160 GJ
Fuel-cycle carbon = 11,630 kg CO2.

BIG surprise: The vanilla car consumes a total of 1041 GJ of energy, of which its production accounts for only 100GJ, or less than 10%. The story for carbon dioxide emissions follow the same lines.


WHOA! CelloMom has always had the impression that making the car takes a lot more energy than driving it, but is hereby definitively shown to be WRONG. From under her dunce cap, CelloMom will now humbly mumble that it always pays to do the math.

So there you are: from a purely green perspective, and considering only the energy and the carbon footprint issue, yes, go ahead, trade in your gas guzzler for a gas sipper.

Obviously, it makes no carbon sense to do this every year, but consider this: While many Americans trade in their car after 4-5 years (about the lifetime of the car loan), the average life span of a car is about 13 years. That means that all those cars that are traded in go on to find second, possibly third, owners, who have presumably traded up from even older cars. Since cars tend to become less efficient as they get very old, your purchase of a new gas sipper contributes to raising the average fuel efficiency of the national fleet.

Before your green-leaning heart starts beating too loudly, it is interrupted by the voice of the greenback. For the purchase of a new car is expensive. Our "vanilla" car had a fuel economy of 24.8 mpg. Suppose you buy a gas sipper that does 50mpg; suppose its total price, after subtracting any trade-in, is $20,000. At today's gas prices of $3.50 / gallon you would have to drive 282,000 miles (= $20,000 / $3.50 per gal / (1/24.8mpg - 1/50mpg) ) to break even.

It tells you that gas is cheap. At $10/gal, the break-even point would be around 100,000 miles; and at $20/gal, it would be 50,000 miles. Even the Europeans aren't there yet.

Final conclusion:
IF you're ready to say goodbye to the car you currently own anyway, by all means downsize (or at least downsize the engine) to a gas sipper and reap the benefits of lower costs for the new car AND collect the feelgood factor for increasing your personal fuel efficiency as well as the national one.
BUT IF you could happily keep on driving your current conveyance, upgrading to a new gas sipper would earn you serious greenie points, but only that; don't do it for the savings, unless your current car has very high trade-in value. There are several calculators, including Edmund's, that help you make the call - but in general you will be disappointed, dollar-wise.

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