It's been a while since I checked my MPG on my 2003 6 cyl, but the last time I did it I was getting about 17-19 MPG combined. That was when I bought it a few years back with 68k on the odometer.
The two OBD readers that I would recommend are the Bluedriver and the OBDLINK MX+. They cost the same. I don't have personal experience with Bluedriver, but a lot of guys on here use them and love them. I use the OBDLINK MX+. I chose it of the Bluedriver for a couple of reasons.
1) I previously had the OBDLINK LX and liked it, though it only has the basic codes.
2) OBDLINK has an easy to use App, but is also compatible with aftermarket apps should you prefer one. Bluedriver is only compatible with it's proprietary app. I felt that should the worst happen and the company go under I would still be able to use my OBDLINK with another app. Please note that this is unlikely in both cases as they are strong companies and both very popular.
3) All of the vehicles that I have with the exception of my gen 1 Xterra and Frontier are listed for the advanced code reading. They are not listed for either of the readers. It's an age thing.
Before buying, double check that your vehicle is listed in their compatibility charts for what you need it for. All basic codes will be readable on both units, but for ABS and airbags etc you will need to check. These can be found on their websites, or you can always try calling their customer service numbers if you have the urge to be put on ignore and listen to the musical stylings of Kenny G for a half hour. 🎶🎷