Time, Money, Gas
Taking my own advice, I kept my Subaru Impreza between 65 and 70 mph on my Thanksgiving trip to the Bay Area, 5 mph less than what I usually drive. There were a lot of cars passing me on I-5 and I-505. My fuel efficiency for the trip was 30.4 miles per gallon, 1.4 mpg better than the EPA estimate, and it took me about 15 minutes longer to make the trip to Mountain View.
This got me to thinking, what is the value of that 15 minutes? If slowing down five miles per hour on the highway improves your fuel efficiency by, say, 2 mpg, is the gas savings worth more than the time "lost" due to the slower rate of travel?
(Algebra teachers out there, what follows is a good real-world problem for your students).
Seeking an answer to this problem, I dusted off the 'ole 9th grade math skills and crunched some numbers.
Let's say you want to take a vacation from Chico to Yellowstone, 910 miles away. At 70 mph, you can make the trip in 13 hours. At 65 mph, it would take you 14 hours, but your hypothetical fuel efficiency would increase from 28 mpg to 30 mpg. Thus at the slower speed you would save 2.5 gallons of gas over the 910 miles.
At $3.40/gallon, this works out to $8.50 savings for an extra hour on the road.
Obviously, in a more realistic scenario 910 miles might be stretched over a few weeks of commuting, and the hour would broken up into smaller chunks (for example, driving the 62 mile stretch of I-5 from Orland to Redding at 65 mph versus 70 mph would take 4 minutes longer).
Also, though reducing one's highway speed is certainly one of the most basic ways to improve fuel efficiency (most conventional cars reach their peak fuel efficiency between 45 and 60 mph), I have mentioned in previous posts that there are certainly other ways to save on gas.
I would be irresponsible if I didn't mention that the impact on one's wallet is by far not the only reason to reduce gas use, as less gas being burned means fewer emissions (= healthier air) and less money being spent on foreign oil.
But for those folks that look at their pocketbook first when making such choices, I ask, at what price will the cost of gas begin to affect how we drive? Or for that matter at what point will the cost of transporting food from the other side of the country (and beyond) affect our choices in the supermarket?
Oh, and FYI, here is the average cost of premium gas per gallon (with taxes) on November 12 in various European countries, according to the Energy Information Administration (U.S. Department of Energy). For comparison, the U.S. average was $3.33 per gallon.
Belgium: $7.88
France: $7.40
Germany: $7.97
Italy: $7.46
Netherlands: $8.39
United Kingdom: $7.87
Back to our original 910 mile trip to Yellowstone, at $7.50/gallon that extra hour of driving would be worth $18.75.
Comments
I've pondered this before and have often wondered if this computation takes into consideration the amount of time the engine is running? If you drive slower it takes you longer to get there, so your engine is on longer and therefore you are polluting longer and burning fuel longer.
While driving at 65 burns fuel faster, driving at 25 burns it longer. So, there must be an optimal speed in the middle somewhere, like 55, or have they computed this in yet?
Dane
Posted by: Dane Langston | December 7, 2007 02:08 PM
Dale,
The way I see it, it is not how long your engine is running, but how efficiently you are burning the fuel. The bottom line is that you need to burn less fuel to get to the same place.
There is an optimal efficiency, depending on the car it is between 45 and 60 mph, when the car is in its highest gear but with the engine working as little as possible. If I wanted to go faster but still be fuel efficient, I would have to add a 6th and 7th gear to my car to keep my rpms down.
But I see your point, and agree that it is confusing. But look at it this way: Say you are on the road for 15 minutes longer over a 300 mile stretch because you drove 65 mph rather than 70. this saves you a gallon of gas. Here's the clincher--At 65 mph and a fuel efficiency of 30 mpg, it would take me about 28 minutes to burn though that last gallon. So by slowing down an adding an extra 15 minutes to my trip, I kept my car off the road for an additional 28 minutes.
Posted by: jeremy miller | December 9, 2007 08:31 AM