The onboard computers don’t actually measure flowing fuel, but rather are equipped with an algorithm — a small computer program — that monitors several aspects of engine performance and then does a calculation of fuel consumption based on that information.

The actual amount of fuel used is almost always less than the engine computer shows, so your experience isn’t unique.

How precise and accurate the engine fuel-consumption figures are depends on the information measured. The more variables considered, the more precise the information output will be. However, certain assumptions will be made that, if not valid in real life, can throw the computer a curve.

There are a couple of methods to determine, fairly accurately, what your engine’s fuel consumption actually is. The first is an old-fashioned method: Top off the fuel tank at the start of a trip and again at the end of it. Simply divide the distance traveled by the amount of fuel needed to fill the tank. Another way is what is sometimes called a “beaker” test. A fixed amount of fuel is put into a container that the engine then draws from. Again, divide the distance traveled by the amount of fuel used.

There are flow meters available that measure the amount of fuel sent to the engine and the amount of fuel returned to the tank. The difference between the two readings is the fuel actually used. Flow meter kits of this type cost about $700 and are usually accurate within 2.5 percent.