Beginning in the 20th century, gas mileage began to be a concern for consumers. Late in the 20th century, fuel conservation became a significant factor for financial and environmental reasons. However, determining good gas mileage depends on a number of factors, including the type of vehicle involved. Driving conditions, the driver and the car's condition also make a significant impact on gas mileage.
Gas mileage is commonly measured in miles per gallon, or MPG. This is the number of miles a car can typically travel on a single gallon of gas. MPG is higher on the highway conditions than in the city, which consumes more gas with its stop-and-go nature. In the early 21st century, gas mileage for cars on the road varied from less than 20 MPG to more than 40 MPG.
GPM is an alternative measurement developed by Edmunds to calculate the number of gallons required for a car to travel 100 miles. For instance, a car with an MPG rating of 12.5 mpg requires eight gallons of gas to travel 100 miles. GPM might be more helpful in determining the true fuel efficiency of a car in relationship to other considerations, such as its cost.
Since the mid 20th century, most automobiles run on unleaded gasoline rather than leaded gasoline. This makes the engines run cleaner, which can also increase gas mileage. The gas' octane rating also affects gas mileage. Octane values determine the rate gas burns in the car engine based on the ratio of fuel contraction to additive. The higher the octane, the slower the burn and the more efficiently the car runs. In this instance, paying a bit more at the pump can result in ultimate savings because of increased gas mileage.
Vehicles that are built on a passenger car chassis tend to get better gas mileage than vehicles are built on a truck chassis. In the late 20th century, sport utility vehicles (SUVs) and minivans became extremely popular among the American public. However, while many typical cars had 20 to 25 MPG ratings, SUVs during the same period often had ratings of less than 20 or 15 MPG. As of the early 21st century, many cars and small SUVs have advertised a combined gas mileage rating of 20 to 25 MPG or more, with even higher figures for highway MPG.
These are designed to maximize gas mileage by alternating between running on a conventional gasoline engine and a supplemental electric engine. In the early 21st century, hybrid vehicles obtained gas mileage ratings above 40 MPG for everyday driving conditions. Gas mileage ratings for highway driving could be as high as 50 MPG for hybrid vehicles. Hybrid cars also have the advantage of running "cleaner," or emitting fewer carbon emissions, than most conventional gasoline-powered cars.
Other circumstances also affect gas mileage. A well-maintained car will get higher gas mileage than the same model car using the same octane gasoline for which the owner neglects the upkeep, such as oil changes and tire pressure. "Lead-foot" drivers and aggressive drivers tend to put more wear and tear on the car--and have lower gas mileage than more careful drivers.
