"It was 15km/L yesterday, but 12km/L today. Has my fuel economy suddenly gotten worse?" Have you ever felt this way while looking at your car's onboard fuel economy monitor? Just because the numbers change, it doesn't necessarily mean your car has a mechanical issue.
To start with the conclusion: onboard fuel economy monitors and the "full-tank method" measure differently, and both have inherent margins of error. To accurately compare your car's fuel economy, it is essential to first standardize your measurement methods and driving conditions. Buying aftermarket improvement products should only come after that.
What is the fundamental difference between the fuel economy monitor and the full-tank method?
The onboard fuel economy monitor estimates and displays fuel consumption based on vehicle driving data and other factors. The full-tank method is calculated by dividing the distance traveled since the last refueling by the amount of fuel used for the next fill-up. For example, if you drove 300km and the next fill-up takes 24L, the calculation is 300 ÷ 24 = 12.5km/L.
However, even with the full-tank method, the amount of fuel can vary depending on where the nozzle clicks off and the tilt of the vehicle. Just because the two methods do not match does not immediately mean there is an abnormality with the car.
Five main reasons why the fuel economy monitor and full-tank method don't match
- Confusing instantaneous fuel economy with average fuel economy: Instantaneous fuel economy fluctuates wildly during acceleration and deceleration. You should be comparing average fuel economy.
- Different measurement start points: Values reset at the start of a local road trip cannot be compared to values recorded after entering a highway.
- Different driving conditions: Traffic jams, hills, air conditioning, payload, outside temperature, and tire pressure all change fuel consumption.
- Different refueling conditions: The gas station, vehicle tilt, and differences in automatic shut-off points affect the full-tank method.
- Short driving distances: Over distances of only a few kilometers or a single tank of gas, minor differences in conditions will show as significant variations.
Five steps to compare fuel economy accurately
1. Check the car's basic condition
Adjust tire pressure to the manufacturer-specified value and check the air cleaner, engine oil, warning lights, unusual noises, and brake drag. If there are any abnormalities, prioritize maintenance or repairs over measurements or considering aftermarket products.
2. Drive on the same road under similar conditions
Compare routes using the same road, the same direction, and similar times of day whenever possible. Record the air conditioning usage, number of passengers, and cargo weight. Do not engage in dangerous steady-speed driving for measurement purposes, sudden acceleration, or driving that impedes traffic; prioritize traffic laws and safety above all else.
3. Reset the average fuel economy monitor at the same location
Decide on a measurement starting point and reset the average fuel economy at that same location every time. Recording the average fuel economy and distance traveled at the end point makes it easier to notice differences in measurement conditions.
4. Keep the full-tank method as a separate record
Record the distance and amount of fuel at every fill-up and calculate it as distance traveled ÷ fuel amount. If possible, use the same gas station and the same refueling method. However, do not "top off" the tank to the brim for the sake of comparison; follow the automatic shut-off of the fueling equipment.
5. Measure multiple times to see the trend
Average the results of multiple trips over the same section, and use several refueling records for the full-tank method to make your assessment. It is important not to use only your single best result as the final answer.
How accurate are fuel economy monitors?
The American Automobile Association (AAA) has investigated the difference between in-vehicle fuel economy displays and actual measured values. The magnitude and direction of the error vary by vehicle and condition. While this test alone cannot define the margin of error for all Japanese vehicles, it shows that you should not take the displayed value as absolute and must standardize comparison conditions.
Reference: AAA: Testing the accuracy of in-vehicle fuel economy displays
When fuel economy worsens, in which cases should maintenance come first?
If the engine warning light comes on, idling is unstable, there are unusual noises or odors, or the interval between refuelings has suddenly shortened, please consult a dealer or repair shop. Issues with the ignition system, intake system, fuel system, or brakes cannot be fixed with additives or bolt-on parts.
If you want to investigate the cause of worsening fuel economy, please see 10 Common Causes of Poor Fuel Economy.
If comparing before and after installing a product, record fuel economy and "driving feel" separately
Record throttle response, re-acceleration from low speeds, vibration, and noise separately as your "driving feel." Even if the car feels lighter to drive, that alone does not prove that fuel consumption has decreased.
NEXT SCIENCE's Power Module 660 Trial Program is a system designed for you to check the driving feel before and after installation on your own car so you can decide whether to purchase it. It does not guarantee improvements in horsepower or fuel economy. You are free to decide not to purchase the product if you do not feel any difference.
Related concepts are also explained in What to check before buying fuel economy improvement goods and How to experience the Power Module 660.
Frequently Asked Questions
Q. Should I trust the onboard fuel economy monitor or the full-tank method?
A. Do not treat either as absolutely correct; perform continuous comparisons using the same method. Onboard monitors are useful for comparing specific segments, while the full-tank method is useful for confirming trends over several refueling instances.
Q. Is it a malfunction if the fuel economy monitor reading drops suddenly?
A. It can drop due to traffic jams, cold starts, air conditioning, or differences in the measurement segment. However, if accompanied by a warning light or poor performance, prioritize a professional inspection.
Q. How can I verify the effectiveness of fuel economy improvement goods?
A. Standardize the maintenance condition and compare multiple runs under the same conditions both before and after installation. Record fuel economy data and subjective driving sensations separately.
Summary | Standardize your measurement method before seeking fuel economy improvements
Rather than being swayed by fuel economy displays, check the condition of your car and record data over the same segments and under the same conditions. This will help prevent overlooking mechanical issues and reduce unnecessary expenses. If maintenance is needed, do that first. Consider aftermarket products afterward.