As gasoline prices remain high and concerns over environmental impacts strengthen, many consumers are increasingly considering fuel economy when it comes time to purchase their next vehicle. Having an up-to-date mileage rating system in place is imperative then, as buyers demand accurate information in the interest of maintaining desired – and prudent – levels of personal fuel consumption and greenhouse gas output.

Inaccuracies of the old system
That said, the changes affecting miles-per-gallon figures developed by the Environmental Protection Agency and going into effect for the 2008 model year cannot come soon enough. The old rating system is seriously outdated, and a common complaint among motorists – especially those who drive relatively high fuel efficient vehicles – is that real-world commuting never amounts to the fuel economy numbers printed on a car’s window sticker.

Having been developed in the early 1970s, the EPA fuel efficiency testing procedure in place through the 2007 model year was clearly ready for an overhaul. Granted, the rating system was revised in the mid 1980s by simply adjusting city mileage down by 10 percent and highway numbers down 22 percent, but the test itself remained unmodified.

The current 31-minute city loop (in a laboratory) simulates a distance of 11 miles with 23 stops and an average speed of 20 miles per hour. Maximum acceleration is 3.3 mph/sec and top speed reaches 56 mph. For 18 percent of the time, the car sits and idles. The ambient temperature is kept between 68 and 86 degrees Fahrenheit, air conditioning is not used and the engine is started when cold.

Highway driving amounts to an average speed of 48 mph over a 10 mile cycle that takes 12.5 minutes to complete. Maximum speed is 60 mph while the fastest acceleration experienced is 3.2 mph/sec. No stops or idling occur, and the ambient temperature conditions match those used for the city test. The vehicle’s engine begins warm and no air conditioning is engaged.

New rating parameters
Since these existing city and highway standards fail to evaluate fuel consumption experienced by drivers in hot and cold climates or those who accelerate and drive faster, the EPA has added three more testing procedures that will combine with the current city and highway measures.

First, a high speed cycle considers quick acceleration, brief spurts of fast speeds and harder braking. During the 8-mile test that lasts 10 minutes, speed still averages 48 mph but a top rate of 80 mph is achieved. Maximum acceleration occurs at 8.46 mph/sec and four stops are included. No air conditioning, a warm engine to start and 68-86 degree Fahrenheit ambient temperatures are maintained. For 7 percent of the time, the testing vehicle is idling.

Next, an air conditioning evaluation raises the ambient temperature to 95 degrees Fahrenheit while running the climate control system. The 3.6-mile run averages 22 mph, lasts 9.9 minutes, tops out at 54.8 mph and includes maximum acceleration of 5.1 mph/sec. Idling occurs for 19 percent of the time, and five stops are included. The engine starts the test while warm.

Finally, the cold temperature test mimics the city evaluation exactly, except that the ambient temperature is set to 20 degrees Fahrenheit. Colder temperatures are tougher on fuel economy as they keep vehicle engines from warming to their most efficient operating temperatures. The batteries fitted within hybrid-electric vehicles also yield energy less efficiently than they would in moderate ambient temperatures.

Calculating adjusted fuel economy ratings
To allow drivers to determine the way in which the new rating system affects existing fuel economy figures, the website offers a comparison tool going back to the 1985 model year. Simply choose your year, make, model and trim to see both old and new ratings. Hybrid electric vehicles especially drop significantly, as the 2004-2007 Toyota Prius, a vehicle that used to achieve 60 city and 51 highway mpg for a combined 55 mpg, now rates at 48 city, 45 highway and a combined 46 mpg.

How the new figures affect the consumer
Throughout 2007 and into 2008, customers cross-shopping new vehicles from both model years should be aware that the window stickers displaying fuel economy will not necessarily be consistent from car to car. Fortunately, the new stickers appear different enough from the old ones, but the potential for confusion remains should a browser simply glance over the ratings. When Toyota introduced the fully redesigned Tundra earlier this year the manufacturer classified it as a 2007 model, a move some analysts believe hinged upon Toyota’s desire to launch the full-size truck under the old – and more forgiving – rating system to compete better with American competitors.