How much does your gas mileage decline -- and how much does your gas cost rise? -- if you drive at 60 mph rather than 50? How about 70 or 80 mph?

Mark Phelan of the Detroit Free Press asks that question. He says that until last week, nobody had quantified the effect real-world speeds have on fuel economy, despite the fact that most people hit those speeds on the highway every day. New research by the DOE --which works with the EPA to generate window-sticker mileage figures -- should help create models to help drivers figure out the effect higher speeds have on their specific vehicle.

And he's got some answers too. He says you should expect a 41% decrease in fuel economy if you're a leadfoot driver who like drivign at 80 mph instead of 50. And that'll cost you $1.38 more per gallon of gasoline, according to the U.S. Department of Energy's website Fueleconomy.gov.

The differences from one vehicle to another are surprising, according to Brian West, a development engineer who worked on the study at DOE's Oak Ridge National Laboratory in Tennessee.

The smallest drop in fuel efficiency was 6.9% from 50 to 60 m.p.h., while one vehicle lost a surprising 26% between 70 and 80 m.p.h. Other vehicles saw their fuel economy decrease as much as 18.3% at 60 and as little as 10.8% at 80.

The study compares 74 vehicles' fuel economy at 50, 60, 70 and 80 m.p.h. All the vehicles were tested on chassis dynamometers using Society of Automotive Engineers standards. DOE tested 24 of the vehicles in Oak Ridge while Chrysler provided data from 50 vehicles it tested at its labs in Michigan. Vehicles of all body styles, drivetrains and major manufacturers were represented.

While the figures will vary from one vehicle to the next, West said you can use the average decreases for rule-of-thumb estimates of speed vs. fuel economy. You can assume a 12.4% decrease at 60 vs. 50 m.p.h., 14% at 70 vs. 60 m.p.h. and 15.4% more at 80 m.p.h. Thus, 41.8% lower at 80 vs. 50.

By using those percentages and multiplying them by the city or highway mileage your own vehicle should be getting, you'll know exactly how much it costs you to make your next trip at 50, 70 or 80 mph. Is it worth it? That's up to you.