here's a lo-tech aid for people who don't have the luxury of an instant FE feedback device.
i'm not sure if anyone else will have use for it, but for what it's worth here it goes...
it's set up in kilometers and liters and lists my fe in l/100km but the system is easy enough to converty to mpg.
the top lists the fuel used wich you can determine if you always fill up untill the pump clicks off
the table lists the distance traveled (in this case in km's), so when you take the value at the top wich represents best the curent fuel used you scroll down untill you find the value closest to your curent distance and at the left you'll find approx your curent FE
so in my case my fuel gauge shows i've used approx 33L of fuel and my odometer says i've done 513 km
the closest values listed are 516 and 508 so i know my
approx FE should be between 6.4 and 6.5 l/100 km
fuelchart - Google Docs
the system works best if you refere to in when you reach one of the "preset" fuel levels... you can print it, stick it to the dashboard for example.
of cours keep in mind traffic is the first thing you should think about so print it big enough and narrow the range down to the range your car perform is so you can see at a glance what the result is