If you go with a ScanGuage, I would recommend passing on the rearview mirror mount as offered here. While it's a great piece of hardware, and a very clean install, mounting the ScanGuage in that location makes it almost impossible to read except at night. The ScanGuage display is not that bright, and with being backlit through the windshield, the display just washes out.
http://www.mgtmotorsports.com/
The ScanGuage itself, I do like, though sometimes it's better not to know your fuel usage and live in denial.
Herb