I heard a segment recently on NPR about gas prices and it reminded me of the link between driving speed and gas mileage. According to the story, driving over 60 miles per hour is equivalent to paying $.25 more per gallon for each five miles an hour faster. So if you paid$4.00 for a gallon of gas, but burn it at 65 miles an hour, it’s like having paid $4.25. Seventy miles an hour makes it $4.50, etc.
It seems funny that it’s very easy to imagine Americans complaining about the price of gas, but very difficult to imagine them driving the slightest bit slower in order to save that same money. In fact, I can easily imagine an American burning a 70-mile-per-hour gallon of gas to go out of their way to save $.25 cents per gallon at the pump. (It could be a good economic decision to do so, I suppose, depending on how many gallons of gas you need to buy, and how much your time is worth, but it still seems funny.)
If I remember my physics correctly, I think the loss of efficiency is actually not that linear, that a 75-mile-per-hour gallon probably costs significantly more than the $4.75 that NPR’s equation predicts. It has to do with the amount of force each particle of air hits the front of our car with–it’s the same principle we (if we are from the desert) use to remember to drive slowly or stop during a sandstorm, to save our windshields from getting sandblasted. Any physicists in the audience care to explain the mechanics of it?
In his excellent lecture “Climate Change Recalculated,” engineer Saul Griffith tells about how he gave an intern this incredibly boring job: Drive his wife’s Honda Insight in 100 mile stretches around a runway at constant speeds, twelve 5-mile-per-hour increments from 20 to 75 miles per hour. Seventy-five miles per hour was the worst, obviously, at about 40 miles per gallon, and the most efficient speed, at about 85 miles per gallon, was 30 miles per hour.
That’s pretty slow, but three times as fast as the average driving speed for large urban areas, he points out. I’ve been thinking about making my next trip to Portland at 30 miles an hour, to see how little gas I can use to get there. It’ll take 4 hours to get there, so I’d better bring some good company.
May 28, 2011 at 3:03 pm
I’m game, but are there roads where you’re allowed to go that slow the whole way?
May 28, 2011 at 4:31 pm
Right. We might get a ticket for “impeding traffic.” I wonder how much you pay for a class D violation? Might be worth the experiment. Maybe we could get away with it in the middle of the night.
And there probably are routes that would be better than the freeway. I’ll look into it.
May 30, 2011 at 10:15 pm
You probably could just go up 99. We used 99 between Corvallis and Eugene when we were having car trouble.
May 29, 2011 at 2:36 pm
Kinetic energy = 1/2 Mass * Velocity^2 . When you hit those high speeds, the velocity factor kicks in big-time (comparative to mass). At least that’s what my 10-year-old physics brain tells me.
June 6, 2011 at 10:17 pm
I just saw some interesting data on this last week in a seminar on the relationship between traffic congestion and emissions.
The presentation is available streaming – June 3rd, Alex Bigazzi.
http://www.cts.pdx.edu/seminars/index.php
February 8, 2014 at 11:51 am
[…] I decided on an experiment based on Saul Griffith’s (which I wrote a bit about and linked to here): I drove a full tank with an self-imposed speed limit of 60 MPH and then a tank at 55 MPH max. […]