GPS outta question, as of now. (Waiting for the next best sexiest GPS phone, which will be cost effective - it's like chasing a mirage!)
Cross-check with odo is a good idea, I'll do that over the weekend where I travel at least 30 miles in one go, so as to bring out small differences if any.
OT: My regular trips are 5 miles one way, which are a big let down for FE - the engine is barely warm by the time I pull up in my office parking lot. And that makes me realize hypermiling can mess you up in the head - making you yearn your average trip length were more so as to score better on FE
