Most people know vaguely that if you drive faster on the highway, you’ll pay a price at the gas pump.

A fast road trip costs more than a slow one because it burns more gas. But how much more, exactly?

How do you calculate the time-is-money tradeoff between miles per hour and miles per gallon?

Three engineers at the U.S. Department of Energy’s Oak Ridge National Laboratory have worked out some answers. And now, the Road Worrier will try to translate their computations into practical English.

Here’s one way to make sense of their findings:

Say you could drive 60 mph on your next trip, but you prefer instead to drive 70 mph. You’ll pay 16.3 percent more for gas, Oak Ridge says. At today’s average price, that’s like spending an extra 55 cents a gallon.

How you like them apples?

“It really strikes home,” said Garry Kenney, a former Raleigh computer networking consultant now retired in Davidson County. He drives a Prius and pays attention to fuel economy.

He knows lots of drivers who worry about the price of gas.

“They’ll run all over town trying to find 5 cents a gallon difference in the price,” said Kenney, 69. “But, hey, how fast you drive makes a bigger difference than 5 or 10 cents a gallon.”

Oak Ridge is where they figure out the fuel economy ratings for new cars. In this exercise, they tested 74 new and old cars and trucks at 50, 60, 70 and 80 mph. For every 10 mph increase, they charted the corresponding decline in mpg.

The results varied a bit from one car to another. The fuel economy drop-off was sharper when cars accelerated from 70 to 80 mph (15.4 percent) than when they went from 50 to 60 mph (12.4 percent).

The best speed for top fuel economy varies from one car to another, said Brian H. West, one of the Oak Ridge engineers. Some drivers like to think their cars perform best in the fast lane, but that’s not likely.

“Certainly nothing we tested here is better at 70 mph than at 60,” West said. “And the fuel economy at 60 mph was always lower than the fuel economy at 50.”

It’s basic physics, if you must know.

The aerodynamic drag pushing back on your car increases with the square of your speed: If you double your speed, the drag increases fourfold. And the power required to overcome drag increases with the cube of your speed. To double your speed, you need an eightfold increase in power, West said.

Satisfied? Now let’s get back to your road trip.

Speed has its benefits, of course. If you set the cruise control at 70 mph instead of 60 mph, you’ll cut your travel time by 14.3 percent. On a 100-mile trip, you’ll arrive 14.3 minutes sooner.

Let’s say your car gets 25 mpg when you drive 60 mph. That’s 4 gallons for 100 miles. The Oak Ridgers say that at 70 mph your fuel economy will fall by 14 percent, to 21.5 mpg. Now you’re up to 4.65 gallons.

This means you’re burning 16.3 percent more gallons per mile.

Monday’s average price for self-service regular gas in the Triangle was $3.38 a gallon. If your fuel cost rises for that fast trip by 16.3 percent, it’s like paying $3.93 a gallon. (Want to drive 80 mph instead? Make that $4.65 a gallon.)

Suddenly, speed carries a price. Getting there 14.3 minutes faster costs an extra $2.20.

OK, we’re done with math. I should note that we’re talking here only about the fuel costs that come with high speed. More cars crash and more people are hurt when we drive faster, but that’s another story.

West likes the idea of looking at this in terms of gas prices. It’s easy to measure the time you save by driving faster, he said, but hard to gauge the expense.

“I guess that’s why people drive fast,” said West, deputy director of the Fuel, Engines and Emissions Research Center at Oak Ridge. “They don’t perceive the cost as being that significant.”

Although fuel economy is his business, he’s not a slow-speed hard-liner.

“Driving slower can certainly save gas,” West said. “But if you’re on the interstate and everybody is going 75 mph and you want to go 55 mph, you’re putting yourself at risk. Of course, we recommend everyone not exceed the speed limit.”

Now that Gary Kenney is retired, he can afford to take a little more time on highway trips.

“I’m not in a hurry, most times, to get somewhere,” said Kenney, who lives at Badin Lake. “So I’ll try to drive near the speed limit.”

His car has a fuel economy gauge that rises higher when his speedometer falls lower.

“I’m conscious of it. And it makes a difference I can see,” Kenney said.

Make contact: 919-829-4527 or bruce.siceloff@newsobserver.com. On the Web at twitter.com/Road_Worrier/ and blogs.newsobserver.com/crosstown/. Please include address and daytime phone.