I have a 2002 Focus Wagon 5-speed DOHC. I bought the EconoDriver from Lamurmonitors. It is a simple aftermarket MPG monitor. It was $19. Easy install- plugs into the OBD connector and is wireless. I have monitored my mileage for about 8 months. I get 29 mpg in city and $33-36 mpg on the interstate. I have gotten 37mpg a few times.
I have found that 10% ethanol gas works best. I keep the car very well maintained. The first 10 min of driving are the worst- then mileage improves. I drive 75 mph on the interstate. I do see if I get behind a semi my mileage goes way up. I don't have to tailgate just use them as a wind block.
I like the monitor. It has a 5 leaf indicator on the monitor that has taught me the best driving patterns. Besides changing all the filters, and I changed the gas cap, tires at 32 psi, and I run Lucus fuel injector cleaner every 5- 10 tanks- I have been very happy with my mileage. I highly recommend the monitor. And It is portable so I use it on the other cars as well.