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Fuel Efficiency Versus High Mileage Ratings, Part II

Charles Krome
by Charles Krome
August 6, 2009

In yesterday's column, I got on my soapbox about the difference between a car that gets good mileage and one that's fuel efficient. Basically, the concept was that a vehicle like the Ford Taurus, which weighs in at 4,015 lbs. and gets a combined EPA number of 22 mpg, is actually more efficient than a Mazda MX-5 Miata, which tips the scales at just 2,480 lbs. and gets 24 mpg combined.

Well, after my fumbling attempts to quantify this previously, I put the crack Krome on Cars research staff to work and here's the new, improved formula to back this up: First, you figure the gallons per mile number, to tell you how much fuel it takes to push a vehicle 5,280 feet. Then, you divide by the vehicle's weight to determine how much gas is needed to push one pound of that vehicle a mile. Now, that number can be compared across different vehicles to see how much gas is being used to push the same weight (1 lb.) the same distance (1 mile). The car that uses the lowest amount of gas is pushing more efficiently.

So, the Ford uses about .045 gallons of gas to push 4,015 lbs. a distance of one mile. Divide by the weight of the car, and you get .00001132, which is the number of gallons of gas it takes to drive 1 lb. of the Taurus one mile. (But in the same way a batting average of .300 is called hitting "three hundred," you can drop some decimals and say the Krome score here is 1132.)

To come back to the Miata, well, it ends up with a Krome score of 1680 '” and that's not really too good. A MINI comes in with a score of 1227, which reflects the fact that it weighs a bit more than the Mazda (2546 lbs.) but gets much better gas mileage (a combined 32 mpg).

Again, this essentially reflects how much gas is needed to move one pound of each vehicle one mile.

The results are pretty fascinating, especially that H2/ForTwo comparison, which show the huge HUMMER is actually more efficient than the not-so-smartly built Smart. And by the way, the H2 is so big that GM doesn't have to report its mileage, but a quick survey of the online experts indicates it gets around 10 mpg, so that's what I used in tallying its Krome score here.

Now, the name of the game is still, ultimately, to decrease fuel consumption, but these numbers can go part of the way to showing how much is still on the table for a given vehicle. Again, let's look at the H2 and the ForTwo. The reason the latter is less efficient than the former is that the Smart may get a combined 36 mpg, but it only weighs 1,808 lbs. From a physics standpoint, it naturally takes less fuel to move that amount of weight; if the overall Smart package were as efficient as the Honda Fit package, the German city car would get 42+ mpg. Which should be completely doable, because the Fit itself is proof that a car that efficient can actually be built.

And I think that's probably enough math for the day '” perhaps the entire year.


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