I have a 2000 Ford E350 with a V10. I am looking at adding a fuel monitor. I've heard they are easy to put in by just plugging into the OBD2 connector. By past experiences can someone recommend a name brand or model.
MPG gauges that plug into the lie-o-meter, err I mean computer, are notoriously generous on their readings. In stead consider a fuel flow meter. I got the idea from mythbusters. In stead of calculating mpg, it measures GPM (gallons per min). While no, it wont tell you how many MPG you are getting, but it gives a 100% accurate relative reading on how much fuel is being used. And some quick math will give you MPG..
I've never seen one is a normal auto gauge fashion. (ie, autometer, issopro, etc.) However, you can order them or get them at lowes. I've even seen some with digital readouts, which could be installed somewhere in the dash.
Its definitely something you would have to custom make. I've never seen one purpose built for measuring fuel flow to measure MPG.
This is just a question, but wouldn't a vacuum gauge be the most 'accurate' fuel-being-used-at-the-current-time meter?
Unfortunately, Trey is correct (although the new 2011 F150 dash display meters seem to be a lot closer than previous versions) about the dash or OBDII meters (including my Gryphon/Edge CTS on my 2009 F150).
Only way to be exact is to maintain a mileage and fuel used log (kind of driving by looking in the rear view mirror). I keep mine on Fuelly.com (as well as a spreadsheet).