Originally posted by scratchpaddy I used to use a bluetooth OBDII dongle and smartphone app on my Metro to do the same thing. It was amazing how much real-time data you could get from a car made in 1996! Also addicting, to keep track of all those data points to try and see how you could get just a little bit more efficiency out of it. I've calmed down a bit since then. The Mirage doesn't have anything colorful or animated to monitor efficiency like many other cars now, but it does have a basic cumulative MPG display on the trip meter. It's only off by about 5% compared to the real mileage-to-gas-pump calculations (some other car's efficiency displays are wildly optimistic). I still enjoy trying to keep that number up on road trips.
My mileage on summer trips is always better than in the winter, in spite of the air conditioning. Something to do with the density of the air? I always get more mileage at higher elevations, too, even with the lower-octane gas they sell there.
Cars have had electronic control modules since the late 1970s. Sure they were stand alone "dumb" systems, but data could be extracted real time if a person wanted to go to the trouble.
Today the fuel economy numbers are extracted from the pulse width modulation of the fuel injectors. The ecm calculates how much fuel passes, compares that to vehicle speed, distance traveled, throttle position, and engine load (calculated by either the mass airflow sensor, a manifold absolute pressure sensor, or a barometric pressure sensor depending on the configuration).
When calculating fuel economy manually, even then there will be error, as one cannot always refill the tank with exactly to same amount each fill-up.
When I worked at PACCAR doing product test and development, we ran fuel economy testing on heavy trucks. A calibrated fuel tank was used, the fuel was weighed before and after each phase of the test, atmospheric conditions were monitored, recorded and factored into the fuel consumption. The trucks were outfitted with calibrated speedometers and odometers (in addition to a wide array of other testing and recording instrumentation), and the route was carefully planned and every effort was made to duplicate the conditions.
We even weighed the truck, trailer and load before and after each test.