How is the 100-kilometer fuel consumption of a car calculated?

[CamilleRose]

The calculation formula for 100-kilometer fuel consumption is: 100-kilometer fuel consumption = fuel refill amount / (kilometers driven after refill * 100). 100-kilometer fuel consumption refers to the fuel consumption of a vehicle traveling 100 kilometers on the road at a certain speed. It is a theoretical indicator of the vehicle. For example, the Xingyue L has different 100-kilometer fuel consumption for different models: Xingyue L-2021 model-2.0TD-DCT-EVO two-wheel drive Luxury, Flagship, Premium, and Intelligent Premium versions (5 models in total) have a fuel tank capacity of 55L. A full tank can run for 808km, and the 100-kilometer fuel consumption is 55/808100=6.8L. Xingyue L-2021 model-2.0TD high-power automatic four-wheel drive Flagship and Premium versions (2 models in total) have a fuel tank capacity of 62L. A full tank can run for 795km, and the 100-kilometer fuel consumption is 62/795100=7.8L. The 100-kilometer fuel consumption is a value measured by the manufacturer in an objective environment using a dynamometer installed on the vehicle's chassis, converted into speed parameters, and then calculated under specified speeds to obtain the theoretical experimental 100-kilometer fuel consumption data for the model. The level of a car's fuel consumption is directly related to five major factors: driving habits, the car itself, road conditions, natural wind, and environmental temperature. Specific factors that increase fuel consumption are as follows: Driving habits: Aggressive driving, such as long-term high-idle low-speed driving, sudden acceleration, frequent overtaking, and not releasing the throttle in advance when encountering a red light, will increase fuel consumption. The car itself: Cars with larger displacements consume more fuel than those with smaller displacements because larger displacements generally mean higher power, requiring more gasoline to burn for work. Cars with greater self-weight consume more fuel because greater weight requires more driving torque. For example, SUV models consume more fuel than sedan models. Road conditions: Dirt roads, muddy roads, soft surfaces, mountain roads, etc. Driving on these surfaces increases resistance and fuel consumption. Natural wind: Driving against the wind or on windy days, opening windows at high speeds increases wind resistance, thereby increasing the car's resistance and fuel consumption. Low environmental temperature: When the engine block temperature is low, the gasoline injected during a cold start is not easy to atomize, requiring more gasoline to be injected for combustion, thus increasing fuel consumption. At the same time, the engine computer will control the use of higher RPMs to warm up the car in low temperatures, which also increases fuel consumption.

[ODante]

I've been driving for ten years, and calculating actual fuel consumption is quite simple. Every time I refuel, I fill up until the pump automatically stops, and then reset the trip meter on the dashboard. After driving 300-400 kilometers, I refill the tank and note how many liters were added. For example, if I added 36 liters after driving 450 kilometers, dividing 36 by 4.5 (per 100 kilometers) gives 8 liters per 100 kilometers. This method is much more accurate than the car's onboard computer. It's important to avoid testing during heavy traffic or after aggressive driving on short trips; the best conditions are regular commuting routes. I've noticed that my fuel consumption drops by over 1 liter when I drive on the highway weekly, but it increases when using air conditioning or carrying a full load in the summer.

[MacViolet]

I'm always fascinated by how those numbers in cars come about. The onboard computer calculates fuel consumption using three sensors: the mass airflow sensor measures the amount of air intake, the injector pulse width estimates fuel injection quantity, and the wheel speed sensor calculates mileage. However, the ECU often inaccurately averages idle fuel consumption or short trips, showing 6.5 liters when it might actually be 7.2 liters. For precise measurements, you need to conduct your own tests. I make it a habit to record data in an app every time I refuel, and after driving 5,000 kilometers, I can plot a fuel consumption curve. I've found that a 0.3 bar difference in tire pressure can cause a 0.6 liter fluctuation in fuel consumption, and higher oil viscosity also increases fuel usage.

[VanWalter]

When I was a beginner, I also couldn't understand the fluctuating fuel consumption numbers on the dashboard. Later, a mechanic told me the simplest method is to wait until the fuel gauge lights up yellow, then fill up the tank and note the amount of fuel added. The next time the light comes on, check how many kilometers you've driven. Last month, my fuel tank warning light came on, I added 42 liters, and this time when the light came on, the trip meter showed 520 kilometers driven. The calculation is 42 ÷ 5.2 ≈ 8.07 liters per 100 kilometers. Note that when the light first comes on, there are still about seven to eight liters of fuel left in the tank, and the remaining amount varies by vehicle model. Using an app to record a few times will help you understand the pattern. This method is most accurate for measuring long-distance trips.

[LaSophie]

The essence of fuel consumption calculation is a physics formula: Fuel consumption per 100km = Total fuel consumed ÷ Total distance × 100. Last week when I helped a friend inspect a used car, I specifically conducted three tests: the first was a cold start followed by 20km highway driving, showing 7.8L; the second was 15km in congested traffic showing 9.3L; the third was a comprehensive road condition test with an actual measurement of 8.4L per 100km. I found that the short-term data error of the car's computer can reach up to 15%, especially when there are frequent starts and stops. I recommend car owners use the full-cycle testing method: record the mileage and total fuel refilled at the beginning of the month, and at the end of the month, divide the total fuel consumed by the distance traveled multiplied by 100, which includes all road conditions for the most objective results.

[StKira]

Many people are concerned about the inaccuracy of displayed fuel consumption, but in fact, the calibration logic varies by manufacturer. Japanese cars tend to include idling fuel consumption, while German cars often exclude fuel used while parked. During actual measurements, it's important to note: continuing to refuel after the pump automatically stops can lead to calculation errors; false tire pressure alerts can significantly increase rolling resistance; even the heat value difference between 92 and 95 octane gasoline can affect consumption by about 0.3L. Personally, I make it a habit to fill up the tank at the beginning of each month and take a photo of the odometer. At the end of the month, when refueling to full again, I divide the mileage difference by the amount of fuel added. After six months of tracking, I found that winter fuel consumption is 12% higher than in summer. This long-term data is much more accurate than single measurements.