This is on our road map. The plan is to allow you to fill up the fields that you know. So if you have gallons and price/gallon, that's all you need. If you have total cost and gallons then it will divide to get price/gallon. If you have total gallons and price per gallon it will divide and fill in total cost. This logic will be first available from the native smart phone apps (iOS/Android) and later on the web based form.
@DTMAce - WE also plan to allow you to attach photos to fuel-ups so snapping a photo of the pump is a great way to ensure you have even more records of your data. We are also playing around with the idea of OCR so we can convert your photos and the fuel-up for you automatically. We think there is a lot of value in snapping photos at the pump, especially when users might walk around and take some photos of the vehicle which help document it's condition over time. Lots of neat things to think about regarding that.
I suggested having the ability to enter total price and quantity purchased some time ago, but it was shot down. I don't habitually record unit price, nor do I enter data at the point (or time) of fueling. It is a nuisance to divide total cost by number of gallons, enter unit price and gallons, then watch Fuelly calculate the total for me.
Speaking of nuisance factors, it is a nuisance to have to scroll down on the Add Fill page to complete the thing. There is enough wasted real estate on the page that the whole thing could be done without scrolling.
I think this is a great idea! Entering the total cost is not harder than entering the price / unit, in fact, it is easier sometimes.
I live on the border and fill up in US and Canada, and sometimes have to convert $/L into $/Gallon, or vice versa, so I gave up and leave the price blank when I have to convert. Looking forward to when you guys add the feature in the future!