How to Calculate Fuel Cost

A customer prepares to fill his car with gas at a station in Zelienople. Pa.
A customer prepares to fill his car with gas at a station in Zelienople. Pa.
AP Photo/Keith Srakocic

Good fuel economy isn't considered an afterthought when it comes to buying a car anymore. These days, the ability to get good gas mileage will often be the main reason someone buys one vehicle over another.

And why wouldn't it be? With gas at more than $3 a gallon in some areas, it's no longer cheap to fill up. In addition, drivers are more aware of the environmental and political impact of their vehicles than ever before. Today, most consumers want a car that has low emissions and won't use too much gasoline, a fossil fuel that's in limited supply and often a reason for global conflict.

In other words, fuel efficiency is "in." Drivers are turning to smaller cars, hybrids and clean diesels at levels never before seen. Many cars even include fuel economy gauges that indicate exactly how many miles per gallon they're getting.

But you don't need a new car or new technology to tell you how "green" you're driving. It's easy to calculate your vehicle's miles per gallon, and to keep a watchful eye on how expensive gas is in your area, too.

In this article, we'll discuss how to calculate your own personal fuel cost. You'll also learn just how much gasoline you use on a regular basis, and how to find out whether or not you can expect gas prices to go much higher.

Calculating Your Vehicle's MPG

A customer pumps gas into her car at a gas station in San Jose, Calif.
A customer pumps gas into her car at a gas station in San Jose, Calif.
AP Photo/Marcio Jose Sanchez

In commercials and advertisements for cars we often see MPG, or miles per gallon, featured prominently. Most of us know that a higher MPG is more desirable because it equates to less visits at the gas station. But what does that number really mean?

Essentially, MPG tells you how many miles your car can travel on a single gallon of gas. If, for example, your car gets 34 miles per gallon, then traveling 34 miles should consume exactly one gallon of gas.

We usually see two different figures -- city MPG and highway MPG. These figures are determined by the federal Environmental Protection Agency, which rates the fuel economies of new cars after rigorous testing on a dynamometer. City MPG refers to driving with occasional stopping and braking, simulating the conditions you're likely to run into while driving on city streets. Highway MPG is based on more continuous acceleration, which usually yields a higher figure because it's a more efficient use of the engine.

However, these figures aren't necessarily the same as what you would get in everyday life. Everyone drives differently. These tests don't take into account how fast a driver drives, when they choose to shift gears (if they use a manual transmission), or road conditions unique to your area. In other words, your results may vary.

Here's how to figure out your own personal MPG: First, fill your gas tank and then reset the trip odometer. Drive until your car's gas tank is nearly empty, or close enough to it that you need more gas immediately. Look at how many gallons of gas it took to fill up the tank completely. Now take the reading on the odometer -- the number of miles you've driven since your last fill up -- and divide it by the number of gallons used.

In other words, if you have an empty tank and put 10 gallons of gas in it, then see that you traveled 350 miles, you're getting 35 miles per gallon. Over time, this will give you an accurate measurement of how efficiently you drive your car in real-world situations.

Finding the Current Gas Price

High gas prices are posted at a Shell gas station in Menlo Park, Calif., in 2008.
High gas prices are posted at a Shell gas station in Menlo Park, Calif., in 2008.
AP Photo/Paul Sakuma

Lots of factors can affect the price of gasoline. International supply, taxes, the time of year and even speculation can all cause the price of crude oil (which is processed into gasoline) to fluctuate. In addition, gas is more expensive in some parts of the country than others. California, with its high taxes and large metropolitan areas, often has higher gas prices than the rest of the nation.

For these reasons, it's good to know how expensive gas is locally. If you're traveling to a certain location, it can be helpful to your budget to know approximately how much money you'll need to fill up your rental car. It's also good to know where you can get cheap gas in your own area.

There are many ways to determine the current gas price. AAA runs a Web site that keeps a daily report on fuel prices across the country. You can use it to find the national average, and to search for your own state. At the time of the writing, the site shows that West Coast states are getting hit with the highest gas prices, while states near the Gulf Coast have the cheapest gasoline.

MapQuest also lets you search for gas prices around your local area. Just enter your address and the results will show you a list of gas stations around you, along with how much you'll pay at the pump.

The EPA also maintains a list of links that allow you to find gas prices locally. This is a great resource if you're going to be traveling and can't decide whether your rental car should be a Corvette or perhaps a Prius.

There are many other great resources online also. Smartphones like the iPhone and Android devices, for example, are great on-the-go devices for finding current gas prices, too. You might even save some cash if you take advantage of some of the gas price apps they can run.

Will gas prices get more expensive soon? That's a question on all our minds, whether we're buying a new car or planning a big vacation. Find out on the next page.

Finding Gas Price Predictions

A customer fills up his car at a Speedway gas station in Columbus, Ohio.
A customer fills up his car at a Speedway gas station in Columbus, Ohio.
AP Photo/Kiichiro Sato

Crude oil is a commodity -- an item that's the same no matter who produces it but the price can fluctuate depending on various conditions. Those who trade in oil futures, or the price of oil at a later date, can have a huge effect on the supply of oil. In addition to futures trading, unforeseen events can have a negative impact on the supply of oil. An explosion at a refinery, for example, or the recent disaster with the BP oil spill in the Gulf of Mexico can hamper the supply of oil and drive up its price.

One way to predict whether gas prices will increase is to monitor the trading of crude oil on the New York Mercantile Exchange, or NYMEX, the market where commodities are traded in the U.S. You could also get news from a service like OPIS, the Oil Price Information Service, or several other sources of information on the oil market.

Many news services or web sites claim that gas prices are nearly ready to increase wildly; but take these predictions with a grain of salt. Some of them could be scams or could lead to bad investment decisions. It's best to stick with reliable, proven news outlets. And remember, like any commodity, no one can perfectly predict the future of gas prices. However, staying on top of current trends can be a good way to save money at the pump.

For more information about calculating fuel costs and other related topics, follow the links on the next page.

Related HowStuffWorks Articles


  • AAA. "AAA's Daily Fuel Gauge Report." (July 15, 2010)
  • MapQuest. "National Gas Prices." (July 15, 2010)
  • OPISNet. "Petroleum News and Pricing." (July 15, 2010)
  • United States Environmental Protection Agency. "Links to Gasoline Price Data for U.S. Cities." (July 15, 2010)
  • Vanderwerp, Dave. "The Truth About EPA City / Highway MPG Estimates - Feature." August 2009. (July 15, 2010)
  • Woolsey, Matt. "The most expensive cities to buy gas in the U.S." Dec. 4, 2007. (July 15, 2010)