Friday, June 20, 2008


High Gas Prices Drive Down Fuel Efficiency

We are currently enduring a natural experiment on the effects of higher gas prices. While it has spurred movement toward more efficient technology, it has brought about some severe consequences that will need to be dealt with.

Last month it was reported that driving in the US was down 4.3% in March compared to last year. What everyone missed was that gasoline consumption wasn't. It was down less than 2%.

For the year, gasoline consumption is down little more than 1/2% [correction, DOE now shows close to 1% less fuel consumed for the year].

We aren't using less fuel, we're getting less done with the fuel we are using.

If the most efficient driving was being eliminated, it still couldn't explain the large difference in fuel efficiency. The driving being cut would need to be several times more efficient than normal to have such a negligible impact on fuel consumption. This is not plausible.

Among the reasons: Less efficient fuel mixtures may be being used; People are acting on bad advice. We've known for awhile now that accelerating faster is more fuel efficient (this is even before considering the beneficial effects on traffic), yet people believe the opposite; People may be driving more at high traffic times to generate needed income and be too tired and poor to drive at other times; And, during the economic slow down, communities may be neglecting good traffic management (e.g. not timing traffic lights properly).

We also need to consider whether higher prices will strengthen the movement toward more efficient technology or have little additional effect. (i.e. Has the move already happened and will further price pressure be of no value? After the crunch of the 70's, efficiency contitued to improve greatly despite falling prices.)

Additionally, we need to realize that in the mid-term, our current vehicle fleet and the infrastructure to produce more aren’t suddenly going to disappear. New tech won’t wash out these adverse effects.

[The gasoline consumption data can be found here:

It's in excel format. See U.S. Weekly Finished Motor Gasoline Product Supplied (Thousand Barrels per Day).

The Energy Information Administration defines Production Supplied as their calculation of consumption:

Products Supplied Approximately represents consumption of petroleum products because it measures the disappearance of these products from primary sources, i.e., refineries, natural gas processing plants, blending plants, pipelines, and bulk terminals. In general, product supplied of each product in any given period is computed as follows: field production, plus refinery production, plus imports, plus unaccounted for crude oil, (plus net receipts when calculated on a PAD District basis), minus stock change, minus crude oil losses, minus refinery inputs, minus exports.

More petrol data can be found here:]

previous post

UPDATE: A mistake was made or the DOE revised their numbers. For the year, consumption is down close to 1%, not close to 1/2%.

UPDATE II: Large trucks do get less than 1/3 of the gas mileage of passanger cars (6.7MPG vs 22.9MPG 2005, DOT). However, they make up only 7.5% of total miles driven.(ANNUAL VEHICLE DISTANCE TRAVELED IN MILES AND RELATED DATA - 2005 BY HIGHWAY CATEGORY AND VEHICLE TYPE) If all driving cut was in passanger vehicles, it should still reduce fuel consumption by only slighly less.

Here are the weekly comparisons of 2008 over 2007, 4 week moving average 2008/2007, and the total consumption, year to end of week, 2008 over 2007:

Weekly | 4 Week Moving | Average Total
Week 1 1.12% 0.42% 1.12%
Week 2 0.62% 1.13% 0.87%
Week 3 -0.49% 0.42% 0.42%
Week 4 -1.65% -0.10% -0.10%
Week 5 -2.41% -0.99% -0.56%
Week 6 -0.45% -1.25% -0.54%
Week 7 -0.79% -1.33% -0.58%
Week 8 -0.83% -1.12% -0.61%
Week 9 -1.31% -0.85% -0.69%
Week 10 -0.29% -0.81% -0.65%
Week 11 -1.83% -1.07% -0.76%
Week 12 -1.45% -1.22% -0.82%
Week 13 -1.70% -1.32% -0.89%
Week 14 -1.96% -1.74% -0.97%
Week 15 0.98% -1.04% -0.83%
Week 16 0.57% -0.55% -0.75%
Week 17 -0.77% -0.31% -0.75%
Week 18 -0.35% 0.11% -0.73%
Week 19 -0.65% -0.30% -0.72%
Week 20 -0.78% -0.64% -0.73%
Week 21 -1.18% -0.74% -0.75%
Week 22 -3.85% -1.62% -0.89%
Week 23 -0.80% -1.66% -0.89%
Week 24 -3.54% -2.35% -1.00%

You're assumming that the "less miles driven" data is actually real.

I don't think it actually is.
The NYT link only mentions that brisk acceleration is better for manual transmissions. Any info on automatics? The Prius has both a CVT and an instantaneous mpg readout, and my impression is that slower acceleration is generally better for mpg. (But as a hybrid, the Prius is not a typical case.)
I expect that your "accelerating faster is more fuel efficient" will be right beside the belief that "my Suburban gets better gas mileage at 80mph than it does at 55mps" in the minds of the "faster is always better" driver.

My impression of vehicles with automatic transmissions is that hard acceleration causes them to remain in a lower gear ratio longer. Given that these are the overwhelming majority of vehicle sold in the US, it seems to me that a little research or qualification is in order.
You seem to have misunderstood what the link means by "brisk acceleration" being better than slow. What the "brisk acceleration" link is saying is JUST before you shift gears you should give the engine some gas, not the entire time you are accelerating. That you should give the engine some gas before you shift gears is something that I was taught in standard shift driving lessons. The reason is that you can slow down by shifting gears too slowly (just cause of the says 3 seconds that your not giving gas). Then would have to give more gas to get back to (and past) the speed you had reached before you shifted. So giving a little extra gas before shifting and shifting rapidly keeps you from slowing down too much.
It's a two part quote. Both are correct.
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