Q: What are the downsides of tracing fuel mileage using the ECM data in regards to best accuracy possible?
A: The most accurate MPG data still comes from using the actual gallons purchased and mileage. The difference between the MPG reported by the ECM and fuel purchased may be small, but important to watch as the two can differ by as much as a full tank of fuel until the driver fills the tank again. Comparing the two sources can help to easily isolate sudden drops in mpg. which may also be a result of theft.
Since the early 1990s fleets have had access to vehicle and driver performance data through the vehicles electronic control module (ECM). It broadcasts data through the J1708 and J1939 vehicle networks. The downfall then was this data was only accessible through a physical link when the truck was back in the shop. Today, many fleets use mobile onboard computers with high speed wireless connections which allows for continuous monitoring of driver and vehicle performance. The ECM contains the most detailed source of information for fuel efficiency, but it is not the most accurate method.
The ECM calculates fuel consumption based on formulas and operating data from the engine and not the amount of fuel injected into the cylinders.
MPG itself is an overall average that is not very useful. There are a number of mobile onboard computer systems which allows a fleet manager to perform field data analysis in "real-time". Fleet managers can accomplish a number of tasks to improve their fuel consumption without sacrificing quality of service. With the information provided they can increase stops per paid hour, consolidate deliveries to reduce consumption and kilometers driven, monitor speed and idling time through these systems and make adjustments on the fly. In addition to route and vehicle adjustments these systems can reduce unnecessary driver hours, improved service to customers through more timely and accurate delivery cycles. Some systems will also include the number of gallons consumed by route, by speed range, by weight(ton-miles per gallon), or when the cruise control is being used. The mobile onboard systems on the market today have proven to reduce delivery costs by up to 15% much being from fuel savings.
I hope this information is helpful to you Don.