Understanding Instant Fuel Consumption Display

Real-Time Fuel Economy in Miles Per Gallon

Close up of a fuel gauge
Ghislain & Marie David de Lossy / Getty Images

Whereas the fuel consumption display (FCD) gauge expresses average fuel usage over a distance, the instant fuel consumption or instant fuel economy display gauge presents a vehicle's fuel consumption instantly, as it is used. As the vehicle moves along, sensors continuously detect fuel flow rate, throttle position, engine speed and manifold pressure. Meanwhile, the onboard computer calculates results and displays them as miles per gallon or kilometers per liter, in real time, to the driver.

The advent of the instant fuel economy gauge came in the late 1990s and was implemented into most vehicles released after 2004 (and many sooner). This gauge uses a complex system of calculations to determine meter read-outs in different parts of the engine that affect its overall miles per gallon ratio. 

Fuel Economy Versus Instant Fuel Consumption

Although the United States Environmental Protection Agency issues regulations on what can be considered a vehicle with good fuel economy, the instant readout gauge determines how well the engine is using its fuel to generate power and how far that power is taking the vehicle down the road. The two terms are not the same, though, so be cautious when referring to "instant fuel economy" when you're talking about instant fuel consumption. This common misconception is a cornerstone of a car salesman's pitch, especially during those test drives! 

Anyway, the instant fuel consumption display is able to compute exactly how many miles the vehicle is able to drive per gallon of fuel consumed based on the car's consumption at that exact moment. Sensors around the vehicle calculate the engine speed, fuel flow rate, throttle position and manifold pressure. When you are looking at the instant miles per gallon reading in your car, you will notice that when you press on the acceleration, the number decreases as you are then using more gas to go faster.

What Is Considered Good Fuel Economy?

When it comes to measuring fuel economy, the EPA calculates the average miles per gallon a particular vehicle is expected to use in its lifetime. However, in personal use and consideration of your car's fuel economy, you must refer to your personal average as the EPA regulations are often based on the "average" driver and you may be better or worse than that standard. That's where the fuel consumption display model comes in, wherein it gauges your personal use and consumption over the course of your vehicle's ownership. 

In any case, the EPA determines a vehicle to be fuel-efficient and to effectively have a good fuel economy if it consumes less than one gallon per 39 miles driven, on average, though the standards differ for cars like the Nissan Leaf, BMW i3 Giga or Toyota Prius Three which all fall under the fuel-efficient hatchback category. Some of these newly designed fuel-saving vehicles get upwards of 100 miles per gallon, significantly reducing gas usage and waste.