2007 Xterra with about 50K miles. I had the service engine soon light come on and the fuel gauge zero out, even after filling full with a new tank. Mechanic charged $400 to replace the fuel injector. However, I seem to getting about 100 miles fewer to the tank now. Would there be a correlation? I used to get 400 miles per tank, now I'm getting 300.

You should stop by a shop or parts store that will get a scan code for free. If you've got code p0463, which is most likely the case in this situation, you've got a problem with your fuel level sensor. As of now, there's a recall on 2006 and 2008 Xterras, but not 2007s. You can do it yourself if you've got the space, tools, and experience to drop the tank and reinstall the sensor which is on top of the fuel tank on the driver side. Taking it to a shop or dealer will run you $400-$600.

Same size tank? Same type of driving? Sounds like a wrong fuel injector burning off too much.