Some Myths Just Won’t Die

Every few months or so someone (usually a non-member) sends us an email that goes something like this:

How can your organization support 65 mph speed limits or even higher? Don’t you know that there’s only so much oil in the ground and that the best way to conserve fuel is to go back to a 55 mph speed limit across the board? Cars traveling at slower speeds use less fuel. It’s a simple matter of physics. Boy, are you guys dumb or what!

It seems that certain misguided notions won’t give up the ghost. So, let’s take a look at the assumptions here.

It is true that a car traveling at a higher speed encounters greater wind resistance (drag) than the same car traveling at a lower speed and that this does decrease fuel efficiency. But that’s only part of the story.

Most experts agree that the optimum vehicle speed for highest fuel efficiency is somewhere between 50 and 65 mph, with most vehicles peaking in the 55 to 60 mph range. The variance is caused by differing engine efficiency and drag coefficients.

When the 55 mph National Maximum Speed Limit (NMSL) was enacted in 1973, federal officials predicted a 2.2 percent savings in gasoline consumption. Instead, the U.S. Dept. of Transportation found the fuel savings to be one percent, and some independent studies determined the savings to be a much lower 0.5 percent. We could achieve those kinds of savings simply by keeping our tires properly inflated.

Further proof that the 55 NMSL had practically no impact on fuel consumption comes from the U.S. Energy Information Agency. In the three years leading up to the passage of the NMSL, U.S. fleet fuel economy averaged 12.0 miles per gallon. The average for the three years following the law’s enactment was basically flat at 12.2 miles per gallon. Only with the onset of Corporate Average Fuel Economy standards in the late 1970s did fuel economy begin to take off.

So, why didn’t the lower posted speed limit produce the promised results? Part of the answer is because posted speeds have little impact on the speeds people actually drive, especially when those limits are set artificially low. (Learn more about how speed limits should be set.) The driving public didn’t modify its behavior enough to make much of a dent in fuel consumption.

In addition, artificially low posted speed limits impede the efficient flow of traffic and further offset any fuel savings. Properly set speed limits based on actual travel speeds promote smoother traffic flow. This minimizes congestion, lane changing, and sudden braking and acceleration, which has a positive impact on fuel consumption (and highway safety).

Steady-state travel on expressways will normally yield better fuel economy than varied speeds on surface roadways. To the extent that a more realistic limit on the expressway draws traffic from the nearby surface highways, the change should yield a small net gain in fuel economy.

The return to 65+ mph highway speeds has yielded a host of additional benefits related to highway safety, economic productivity and time-savings. For example, highway fatality rates have been steadily declining since the repeal of the NMSL in 1995.

For more on the NMSL failure and some practical ways to reduce fuel consumption, check out Jim Baxter’s July 2009 column in U.S. News & World Report.

Not an NMA Member yet?

Join today and get these great benefits!

Leave a Comment

4 Responses to “Some Myths Just Won’t Die”

  1. George_C says:

    Peak Oil is a fraud.
    Oil is formed in the upper mantle, where the temperature & pressure cause bonding of carbon & hydrogen. Long chains are built up, and we end up with crude oil (contaminated with organic deposits), coal-if you find fossils in coal, that implies that coal is not made up of the fossilized element, plants.

    That doesn't mean we should burn it all, because it takes on the order of 700 years for the volume of the ocean to work its way through the faults on the bottom of the ocean.

    How about non-thermal fusion for the power grid first?

  2. Neil Mietz says:

    I have observed that slower is not more fuel efficient. The local state highway is PA 32. It has two speed limits over its length in my area. The road runs along the river and is posted at 35 MPH through town and 40 mph else ware.
    With a 4.0 liter V6 and automatic transmission the tack reads 1750 RPM at 35 mph. At a faster 40 mph the tack drops down to 1175 RPM. So driving 5 mph slower makes the engine run almost 50% faster. (Burn more fuel) I know the transmission shifts to a higher gear about 40, which explains the tack changes.
    My vehicle? A 2009 Mustang convertible. At 68 MPH on the Interstate with the top up I get 28.5 MPH. With the top down fuel mileage drops to 24 MPH. Will they outlaw convertibles or make me drive with the top up?
    Also I can raise my fuel mileage a few MPG if I can use straight gasoline. E10 costs more of a loss of mileage that driving a few MPH faster. And they want to go to E15.

  3. Brother John says:

    If those who write such whiny diatribes want to save fuel for themselves, then let THEM drive under 55. They need to leave me alone; if I pass them, they'll never see me again, ending their involvement in my life. And stay the hell out of the left lane.

  4. PointSpecial says:

    Another thing that isn't being taken into account is that the studies have been talking about the rate of fuel consumption. The difference in the rate of consumption is pretty much negligible…

    But what about the fact that, if you're driving faster, you'll get to your destination quicker? That DECREASES the amount of time that you're burning fuel, thus decreasing the total amount of fuel that you're burning.

    I'm sure there's a way to optimize this (though it likely varies by vehicle), but the EPA has been so focused on the rate of consumption that it doesn't appear that they've even thought about this.