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MPG Accuracy

Solution

Please see the Fuel calibration section of the user manual before reading further.

The MPG, Gallons/hour, Fuel Used, Fuel Level, Time-to-Empty and Distance-to-Empty are all based on fuel usage. UltraGauge uses a variety of vehicle engine sensors along with a proprietary adaptive algorithm to calculate the amount of fuel being used in real time.

The most accurate fuel estimates occur in vehicles equipped with MAF sensors. Vehicles which use the lower cost MAP sensors have less accurate results. However, vehicles with MAP sensors can provide reasonable results when Volumetric efficiency and injector cutoff are enabled and proper calibration is performed. (Please see the user manual for details) Much of what is discussed below applies to MAP equipped vehicles and less so for MAF equipped vehicles.

The estimate of fuel usage is ultimately only as good as the data received by UltraGauge. Each of the various engine sensors has associated errors. For example, Engine temperature may be +-5 degrees and the accuracy may vary over the temperature range(AKA non linear). Also, the sensor data may have poor granularity. For example, the OBDII Mile Per Hour(MPH) data has a granularity of 0.62 miles/hour. So the possible values are 0.62, 1.24, 1.86, 2.48MPH, etc. If the vehicle was traveling at 0.9 MPH, the value presented to UltraGauge could be either 0.62 or 1.24MPH. In this case, the value has potentially significant error at low speeds, and much less at high speeds. The errors associated with each sensor can combine and the resulting accuracy can be very poor. We have seen results as accurate as 0.01% and as inaccurate as +-13%. Your results will vary based upon your particular vehicle, configuration and calibration.

Some fuels have Ethanol and some do not. The accuracy can change if a fuel with a large percentage of Ethanol is used after calibration with a non Ethanol fuel. For example, you may perform calibration locally where simple gasoline is available. Then find less accurate results on a road trip where only Ethanol blends are available.

Also, environmental conditions can affect the accuracy. A vehicle calibrated in summer may see variation in the winter. For this reason, it is recommended to perform the MPG calibration if the environment changes significantly. It is necessary for the Vehicle's ECM to compensate for the environmental changes in order to minimize emissions. Some vehicles compensate better than others. For those that do compensate well, the environmental variation will have a much less significant effect.

Ultimately the MPG gauges should not be relied on absolutely. Instead they should be used in a relative fashion. For example, these gauges can help to maximize fuel economy by maximizing their value at any time.

Note that the above discussion commonly applies to those various makes and model vehicles which are offered with manufacturer's MPG and DTE gauges. For example, Hyundai blanks the digital DTE and Fuel level gauges when the values become small, because they do not want the liability associated with the inherent inaccuracy.

Fundamental to the ability to calculate fuel usage, UltraGauge assumes that the vehicle is in good working operation, such that the emission control system is able to maintain stoichiometric operation.
Please see the description of stoichiometric operation here: http://en.wikipedia.org/wiki/Air-fuel_ratio

If the engine is running poorly or there are engine codes present this could point to the engine's inability to maintain stoichiometric operation, and UltraGauge will not be able to accurately calculate fuel usage. Also, if the manufacturer has chosen to enrichen or lean the mixture under certain conditions, this too will alter the results. For example, a manufacturer may choose to richen the mixture when the air conditioning pump is running at idle.

NOTE! UltraGauge suspends fuel usage calculations while in the Menu system. Avoid running the engine while using the Menu.

 

Diesel Vehicles:  Please see the following: Diesel vehicles

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Article details
Article ID: 15
Category: Knowledgebase

 
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