We took the EPA to task in our April 2013 issue for their trip computer unrealistic fuel-economy estimates of hybrids. Is it possible that the EPA doesn’t just evade fuel economy questions? This is an issue that is very dear to you. I keep track of the amount of gas used and the readings on the odometer. I also record the trip computer’s mpg report. Some variation is normal given the complexities of filling up a gas tank at exactly the same level each time.
However, over the years, I have found that the BMW’s trip computer always reads higher than the mpg calculations, which are based on fuel consumption and corrected odometer readings. The mpg inflation was 0.8-2.6 mpg over the course of 20,340 km and 56 fill ups. Car and Driver has always reported mileage to the nearest whole number. However, because we lack confidence in higher-precision numbers butterfly mobile, Car and Driver made an exception for this study. We are here to help. The BMW’s trip computer averaged 1.4 mpg more–nearly six percent more–than what was calculated.
One Microstep at a Time
Computers control modern engines by calculating how much fuel to inject into each combustion cycle. The computer keeps track of the fuel consumed and distance traveled to determine fuel efficiency.
These potential sources of imprecision exist, but in order to accurately track fuel consumption using your trip computer, you will need to calibrate it [see the below]. The 335i has a service menu that can be accessed via an obscure procedure I discovered on the internet. From that menu, I was capable of changing the trip-computer calibration. The default value of 1000 was changed to 1035, which aligned my trip computer with the reality. Unfortunately, there is no way to correct the BMW’s absurdly optimistic speedometer.
Calibration Station
1) Stop next to a mile marker on your next long journey and reset your trip odometer. After a hundred miles, you can stop again to see how your odometer compares with the road mileage. Next, you will need to reset your trip computer, and then fill up your tank using this method:
- Fill the tank at maximum flow rate until it clicks.
- Wait for 10 seconds before filling again at the lowest flow rate.
2)Repeat the process two more times. This should be done using the same pump at each station. Even with a calibrated Odometer, one fill-up is too many variables to make an accurate calculation. If you log at most three such feelings, you will have enough data to divide your corrected-odometer mileage by the indicated pump gallon to get an accurate mpg. You’ll know your correction factor if you compare the mpg number to what the trip computer reports.
