One of the main ways the automatic transmission can affect MPG is if you have a problem with your TCC, or torque converter clutch. This is a clutch that your automatic transmission uses to eliminate power loss within the torque converter. It works very much the same as a manual transmission clutch and is controlled by the transmission. If this clutch does not engage or it slips when engaged, it can cause a loss of fuel economy. If you have this problem, your engine might run perfectly fine and you’ll have no idea why you’re losing MPG. The one thing you might notice if your vehicle is equipped with a tachometer is that as you’re cruising on the highway, your RPMs are higher than they used to be. This might be because the TCC is not working correctly. The principle is the same as with a slipping clutch on a vehicle with a manual transmission. Here’s a video explaining the operation of the torque converter and the TCC.
Besides the TCC, other automatic transmission issues can compromise MPG. One of those is a slipping clutch or band inside the transmission itself. Most automatic transmissions use clutches and bands to operate the different gears inside of it. If one of these bands or clutches fails, it can cause the transmission to slip in a particular gear. When this happens, it’s a loss of efficiency, and we all know what happens when an automotive system loses efficiency: poor MPG and a loss of power. More information about automatic transmissions and their operation, including some cool videos, can be found in the Solving Transmission Problems article.