When shopping for a new car in the U.S., one of the first questions a potential buyer will ask is, “What is the sticker price?” To answer that question, we first need to ask, “What is the sticker?”
The Monroney label, often called the “window sticker,” is required by law to be displayed in the window of a new car and presents information including the manufacturer’s suggested retail price (MSRP) of the vehicle. The label is named for Almer Stillwell “Mike” Monroney, U.S. Senator from Oklahoma. Monroney sponsored the Automobile Information Disclosure Act of 1958, which mandated the disclosure of equipment and pricing information on new automobiles. The act has been amended numerous times, and the sticker now contains more than just pricing information.
The numbers most influenced by the automotive tribologist also are among the most prominent, namely, the fuel economy and environmental comparisons. In the U.S., fuel economy is traditionally expressed as miles of range per gallon of liquid fuel, either gasoline or diesel. With the growing popularity of alternative fuels (natural gas, ethanol, hydrogen) and propulsion methods (battery electric, fuel cell), presenting fuel economy information in a clear and easy-to-compare fashion presents quite a challenge.
The solution, at least according to the U.S. Environmental Protection Agency (EPA), is to express fuel economy as distance traveled for a given amount of energy consumed. That amount of energy is 33.70 kW•h, which is defined as the energy contained in one standard gallon of gasoline. For any other fuel, the amount of that fuel that contains 33.70 kW•h of energy is defined as the “gallon of gasoline equivalent,” usually abbreviated gge. For example, 0.997 kg of hydrogen contains 33.70 kW•h of energy, so 0.997 kg of hydrogen is one gge of hydrogen.
For the purpose of the Monroney label, the vehicle manufacturer needs to determine how far the vehicle travels, under standard test conditions, on 33.70 kW•h of energy. That number is the miles per gallon of gasoline equivalent, abbreviated MPGe or MPGge. The prospective customer can then directly compare the energy efficiency of different vehicles, even when they use different fuels. Since no driver exactly duplicates the EPA driving cycle, the actual mileage obtained will differ, often substantially, from the posted values. That is why nearly all automobile advertisements feature the standard disclaimer, “Your Mileage May Vary.”
The cost of operating the vehicle depends, of course, on the cost of the fuel. The EPA attempts to assist buyers here by including an estimate of the annual fuel cost on the Monroney label. Since the cost of fuel (and electricity) varies considerably with time and geographic region, these estimates are subject to considerable uncertainty. Nevertheless, it does give the buyer some information on the impact the vehicle will have on their budget.
Of course, most of the world doesn’t express fuel economy in MPG. The most common form is liters of fuel needed to go a particular distance, usually 100 km (l/100km). In this case, a smaller number indicates higher efficiency. So, why does the U.S. continue to use MPG? Perhaps it has something to do with the American obsession with “bigger is better” (as well as a reluctance to switch to the metric system).