Although still in Alpha/Beta stage, I've written a totally
free piece of software for ECU Monitoring that includes real-time Mileage Tracking.
MyMPGTracker.com - ECUTracker
It runs on any Java enabled Mobile Phone or PDA. Or even a dedicated In-Car-PC (this is how I use it). Coupled with a Bluetooth ELM327 (such as the OBDLink), you can very easily watch your real-time MPG values. Or, if you own a Subaru, it also runs the SSM protocol.
Plus, the website that It's hosted on, is a dedicated Mileage Tracking Website.
Also, still in Alpha/Beta mode. But, it is working! The website clearly needs a bit more work. But, it's designed to make it as easy as possible for someone to track a vehicles Mileage. It allows users to send simple TEXT messages from their cell phones to update their mileage info. Or use a mobile phone friendly web-page to enter the info directly.
Anyway... I just thought I'd share my pet project with you Ecomodder Types!!
My endevor to reign in my own fuel costs, hopefully will benefit others as well!
Cheers
Shane.