How accurate is it?

It's as accurate as you want it to be, down to a single day if you want. We recommend taking meter readings every week - this normally gives enough useful data for you to be able to see what's going on. However, you could take a reading every day (at the same time of course) if you wanted to get really really accurate.

If you want the results to be really accurate, consistency is key. Take a reading at the same time every week (or day), or always make sure you fill up your car at the same point on the dial.