Your car’s manufacturer might advertise that it gets 30 miles per gallon on the highway, but can you really be sure of that? Fuel economy is far from a constant, so it does pay to know how much fuel you’re consuming. In addition, chances in fuel economy can be a sign that something in your car needs fixing or replacing. It pays, then, to have an app on your BlackBerry that helps keep track of all that. A featured app in App World does just that. It even has the perfect name for the occasion: Fuel Economy Calculator. This type of app works best from your smartphone, since it’s something you carry with you everywhere. When you pull up to the pump and start re-fueling, you can break out your BlackBerry and fire up Fuel Economy Calculator. Once you’ve added gas, you enter in the amount of fuel you pumped (or, if you’re from Oregon or New Jersey, how much fuel was pumped for you) and the total cost. When you hop back in the car, note the odometer reading. The app will then take that data and analyze it. What you’ll see, after a few fills, is a graph that breaks down your fuel costs and benefits. That is, it will spit out stats like your average miles per gallon, your average distance per tank, your average cost per fill, and other stats. It also graphs fuel prices over time, if you’re interested in that kind of thing. It can be a great tool to help you gain a deeper understanding of what your car costs you. In addition, a drop in fuel economy could mean it’s time to change the air filter, or otherwise to take in the car for a checkup. The app works with both imperial and metric units, so it’s international. It also tracks other car-related items, such as oil change schedule. You might not need to change your oil every 3,000 miles, but you need to do it regularly. You can get Fuel Economy Calculator at App World for $2.99.