Drivers will soon be able to travel at 70 miles per hour on hundreds of additional miles of Pennsylvania highways. Yet those looking to maximize their vehicle’s fuel economy would do better cruising at granny speed.
Speed limits are going up because a broad transportation funding bill that Pennsylvania lawmakers passed in 2013 allowed for the maximum highway speed limit to rise to 70 mph once studies showed the higher speed was safe.
The Pennsylvania Turnpike Commission and the Pennsylvania Department of Transportation announced last week that the speed limit on nearly 800 miles of state highways is being converted, bringing the total state highway miles with a 70 mph limit to just under 1,000.
The agencies studied traffic data and road characteristics, as well as speed and crash statistics in three pilot areas where the speed limit was increased to 70 mph in 2014.
The effect on vehicles’ fuel economy was not one of the features the Department of Transportation studied, PennDOT spokesman Rich Kirkpatrick said.
“Our focus was on safety,” he said.
Turns out most vehicles’ fuel economy decreases at cruising speeds above 50 mph, according to a 2013 study by researchers at Oak Ridge National Laboratory in Tennessee.
They analyzed 74 vehicles of all sorts, including minivans, pickup trucks, hybrids and sports cars. Traveling 70 mph, rather than 60, caused fuel economy to drop an average of 14 percent in the vehicles they studied, all of which were from model years between 2003 and 2012.
Going faster made things worse, with 80 mph speeds decreasing fuel economy by an average of 15.4 percent compared to 70 mph.
“That sounds bad, but when you calculate cents per mile it doesn’t come out to a lot of money,” said John Thomas, an engineer in Oak Ridge’s Fuels, Engines and Emissions Research Center.
The research was used to create a calculator at fueleconomy.gov, which notes that at speeds above 50 mph, every 5 mph speed increase is the equivalent of paying 16 cents more per gallon of gas, based on fuel prices of $2.24 per gallon.
According to the calculator, it will cost less than a dollar more to travel 100 miles at 70 mph rather than at 65 mph in a midsize car at current fuel prices.
“Most people don’t care about saving one dollar on a trip,” Mr. Thomas said, “but we do nationally, because of energy security” and other big-picture concerns.
In a 2009 test on the road, instead of in a simulator, Consumer Reports evaluated the effect of speed on fuel economy by driving several cars over the same route at 55 mph, 65 mph and 75 mph.
“The big difference in fuel consumption was from 55 to 65 miles per hour,” Gabriel Shenhar, Consumer Reports’ vehicle dynamics program manager, said. “The difference between 65 and 70 is not going to be so dramatic.”
The Toyota RAV4 they tested got 29 miles per gallon at 65 mph. At 70 mph, it got 27.5 mpg, Mr. Shenhar said.
“It’s not just about fuel costs. It’s also about emissions,” said Glen Kedzie, vice president of energy and environmental affairs for the American Trucking Association.
The national trucking industry trade group based in Virginia has petitioned the federal Department of Transportation to develop a rule requiring electronic speed limiters on large trucks to be set no higher than 65 mph, for safety, fuel economy and pollution reasons. It has also supported a national speed limit of 65 mph for all vehicles.
Big trucks like 18-wheelers average roughly 6.5 miles per gallon, Mr. Kedzie said. For every 1 mph increase in speed, there is generally a 0.14 mpg penalty in fuel consumption.
Raising speed limits “is counterproductive to what the federal government is trying to achieve now” to reduce greenhouse gas emissions from transportation and across all sectors, he said.
A national speed limit
Four decades ago, fuel shortages and consumption concerns drove the federal government to set a national 55 mph highway speed limit. The national limit was lifted for rural interstates in 1987 and for all highways in 1995. Since then, all states have raised speed limits on portions of their interstates.
Those policy shifts provided perfect test cases to analyze the influence of speed limit changes on things like vehicle crashes, fuel economy and air pollution and its attendant health effects.
Arthur van Benthem, an economist at the University of Pennsylvania’s Wharton School, looked at those combined factors and others. He found in a 2015 paper in the Journal of Public Economics that, for the times and places he studied, the social costs of raising the speed limit from 55 to 65 mph were two to seven times larger than the social benefits, even though the private benefits were net positive for many individual drivers who got where they were going more quickly.
“Driving faster appears a rational choice for many drivers but a poor outcome for society as a whole,” he wrote.
He concluded that the optimal speed limit on rural highways in the late 1980s was slightly lower than 55 mph. In an email, he said his best guess is that the optimal speed limit now would be closer to 65, because cars are now cleaner at higher speeds.
The fueleconomy.gov website, which is administered by the Oak Ridge National Laboratory, does not advocate for a particular highway speed, even though it offers suggestions for increasing fuel efficiency.
“If you were alone in the world,” Mr. Thomas said, a driver could maximize fuel economy by driving somewhere in the 40 mph range, at the lowest speed that is comfortable for the car’s highest gear.
In the real world, “The best speed is when you’re safe,” he said.
“In terms of energy analysis, crashing is a big energy penalty, I’m sure.”
Laura Legere: firstname.lastname@example.org.