You have an old car, gasoline or diesel? So you have problems, like I do, to get an instrument to measure fuel economy in real time?
Try this application,
DriveGain which is free. I am using it and it gave me a very reasonable fuel economy figure.
I use a 1993 Mercedes diesel turbo, 3000cc, which Mercedes Fórums members say will get 12.0 km/l at most. Well, according to the aplication, in the city I am getting about 8 km/l while in the road I got today 12.2 km/l (28.7 mpg), my first road trip since I installed it (30 km). I must say my car is in very good shape and I drove at 80 km/h as a mean.
This App uses only the GPS to calculate yr consumption. You need to enter your car specs, model, engine, fuel, and it calculates yr fuel economy. Mean fuel economy is very good, very reasonable. Instant fuel economy is an option, and works very logical.
In fact, the gps "knows" when you accelerate, when you go down slope, up a hill or flat, and how fast you are driving. With pure physics it estimates everything. Remarkable!
The App has the option to records all yr trips on the web site automatically. So you can review them later. This feature is very good.
Try it and you will be happy.
Oldbeaver