The best I've seen for just pure mileage tracking (there's more to it than math - data entry at a gas pump can be a pain and you want FAST!!) is Automobil.
The best "complete" tracker, with almost too much stuff going on at one time (up to the user), is TealAuto.
I have requested to Tealpoint that PDA Street/Palm Blvd users get a "first crack at customer support" if you mention
paulteal
somewhere in your order or the Attention Line of the email. I have purchased the vast majority of their entire product line over the last couple of years, and in conversation with them over beta testing, almost never fail to mention "this great bunch" we have assembled here and the problems we've all had with OTHER software developers. Teal want to stand out in that department and are EXCELLENT with customer service and help (although reading the documentation WOULD be the best first step!!).
I personally used Automobil for a while, but neither program does you any good in the long run unless you are ADAMANT about entering every single tankful accurately and with a full fill up. The first time you just throw "$10" in because you're running late, you lose accuracy despite "partial fillup" options; it can become a pain after a couple of weeks. I got an estimate over a week's time and then stopped using mileage track programs altogether.
However, for full maintenance record tracking, etc TealAuto is extremely thorough and will even print annual reports for business purposes. Quite complete indeed.