Consumer Reports found that driving 75 mph compared with 55 mph costs drivers an extra $30 on a 1,000-mile trip.

So you dodged the speed traps and cut an hour off your drive from New York to Boston, but you’re still going to pay for that lead foot.

The faster you go the worse your fuel economy, studies have routinely demonstrated, but just how much may come as a surprise. A new study by Consumer Reports magazine suggests that the penalty of driving 75 rather than 55 can add up to as much as 14 miles per gallon – which can add up to an extra $30 in fuel costs over a 1,000 mile trip.

In fact, on long drives, the increased fuel consumption may have an unintended consequence. By burning more fuel you’re likely to stop more often to fill up, the magazine notes, and that means you may lose much of the extra time you hoped to save by speeding in the first place.

The idea that speeding burns more fuel isn’t exactly a new concept. It’s basic physics. It takes more energy to increase your speed as automobiles have to overcome such obstacles as wind resistance and tire friction – never mind the internal losses from the internal combustion itself.

But to see just what all this adds up to in real world conditions, rather than on paper, CR researchers took out five different vehicles: a Honda Accord, a Toyota RAV4 and three different versions of the Ford Fusion, including battery and conventional gas models. They then calculated fuel consumption at 55, 65 and 75 mph speeds.

Jumping from 55 to 75 can cut an hour off a 200-mile drive, but it also can burn as much as two more gallons of gasoline, depending on the particular vehicle, the magazine noted, adding that, “in the cars we used (that adds up to) about $5 to $7. For every thousand miles at 75 mph, you’re increasing carbon footprint by seven to 10 gallons and throwing away about $30.”

Why is 55 the magic number for maximum mileage? If you’re looking at pure physics, it isn’t. That just happens to be the target that was set back in the 1970s, at a time when oil shortages and fast-rising fuel costs led Congress to enact first a 50 mph national speed limit and then raise it ever slightly to the more familiar “double-nickel” that stuck around for years.

The Environmental Protection Agency used that number as the requisite testing point for automotive fuel economy, the source of the mileage numbers on the Munroney window sticker found on every car sold in the U.S.

(Truck sales boom driving down U.S. fuel economy. Click Here to read more.)

The government might have just as well set 50, 60 or even 65 as the test basis and automakers would have responded by specifically tuning their vehicles to deliver the best possible fuel economy at those numbers.

(Click Here to read about the new Corvette getting 30 mpg.)

In fact, data compiled by the Oak Ridge National Laboratory shows that a vehicle moving a steady speed of between 35 and 55 will get roughly the same mileage. The reason has to do with things like transmission gearing. The latest gearboxes, with seven, eight and even nine speeds, are designed to run the engine as slowly as possible when undergoing EPA testing. And that maximizes the resulting fuel economy numbers.

Consumer Reports notes that while driving fast is, well, a quick way to reduce mileage you also cut fuel economy by being erratic on the throttle. Steady as you go is a way to ensure you cut your gas bills.