The thing to keep in mind is that there are two places to set your units. There are viewing units that you set to view across the site. Those are under the Fuelly logo on every page. There are also reporting units that you choose when you create a vehicle. It sounds like you didn't select the correct reporting units when you created your vehicle.
Go to your vehicle page and look for the EDIT CAR link at the top of the page. (Here's a screenshot of the location.) Click that, and then look for the Reporting Units section. Make sure you choose to track in miles instead of km and you'll be set.