I hope you make 500, that would be an achievement!

My best has been 413 on a tank, but I've had several around 400. Those last bars sure do go fast though. You can do 100+ on the first bar and then it seems like the last bars are dropping every 20 miles or so...
My 'good tank' indicators are:
1. Go over 200 miles before half tank (when it drops to 4 bars)
2. Go over 300 miles before it drops to a quarter tank (when it drops to 2 bars)
3. The more miles over each marker, the better (obviously), but that's my guideline