If you go to your vehicle profile page while you're logged in you'll see something called "mini-stats" under the "Basic Stats" section. Click that, and you'll be able to add specific statistics for specific timeframes. For example, you can add "Avg MPG" for the "Last Month". We don't have an option for "Previous Month", but depending on how often you fuel-up, it should be pretty easy to eyeball in your Log Book.
We also show a chart that gives you an average MPG/month once you have at least three months of data. That should give you a quick visual indicator of how your vehicle performs per month.
If you need something beyond these you could always take a look at our
export feature and build your own graphs or calculations in Excel or Google Spreadsheets.