Toyota PriusClick to enlarge picture

Toyota Prius is the most fuel-efficient car in America at about 45 mpg. But using this new calculation, it gets a mere 15 miles per dollar.

For decades, miles per gallon — mpg in its familiar shorthand — has been the only way for consumers to understand how much gasoline is going into their tank, and what it really costs. But mpg isn't cutting it.

Like leagues, fathoms and pecks, mpg has become a relic, a unit of measure that has lost its meaning. It's a quaint reminder of the days when a gallon of high-test cost a buck or less, and Jimmy Carter donned his fuzzy cardigan and kindly asked everyone to crank down the thermostat and conserve energy.

It's time for new rules, and new math: Miles Per Dollar, or mpd.

The formula is simple. Take the old mpg, but divide it by the price of fuel. Unlike the vague mpg, mpd is a remorseless measuring stick, its pointy end aimed directly at your wallet. When gas was cheaper than the dirt it sprang from, 20 miles per gallon seemed pretty solid. Even when gas reached a dollar a gallon, you were still getting 20 mpd, traveling 20 miles on a buck.

The problem is that, as a whole, the nation's cars and trucks aren't getting any better fuel economy than they were 20 years ago. With the price of a gallon of gas soaring to $3 and beyond, think again about that 20 mile-per-gallon car. Its mpd rating has fallen to less than seven.

For a fun new road game, just count the mile markers. Every time you reach seven, you just blew another dollar. If your family hauler gets 7 mpd (or 21 mpg) on the highway, cruising at 70 mph costs a mind-blowing $10 per hour in fuel, more than some people make at their jobs. If that doesn't make you want better mileage, I don't know what will.

Now, a skeptic might call my mpd formula flawed or complicated, because it fluctuates with the price of gasoline. But that's precisely the point. As gas prices pole vault to new heights, mpd grabs you by the collar and says, "Um, buddy, have you seen how much you're spending to drive to Dairy Queen and back?"

Miles per dollar: Great way of calculating fuel use or not being realistic?

It doesn't help that the government and car companies have fudged on miles-per-gallon ratings for years, confusing and misleading consumers. The Feds finally created a more realistic real-world mileage test that's in effect for 2008 models. Under the new test, a four-cylinder 2008 Honda Accord — an efficient sedan if ever there was one — gets 21 mpg in the city, 31 highway, and 24 mpg overall.

Put that Accord under my mpd microscope, and you're getting just 8 miles per dollar. At 3 bucks a gallon, the Toyota Prius, the most fuel-efficient sedan sold in America at about 45 mpg, manages a mere 15 mpd. If gas hits $4 a gallon, even a Prius will be burning through a buck every 11 miles.