Something like Fuelly
 

Is there a website like Fuelly that actually works?

When I try to add a fill up, there is no combination of dropdown selections for model and engine, etc, that satisfies it, so Im in an infinite loop. Yet I can't just tell it what my vehicle has...

There has to be something better out there.

You set your vehicle up through the dashboard "add a vehicle" button? Once you do that you don't enter a model and engine when you add a fill up. It already knows the details of your vehicle. You just input miles, gallons and price. What year make and model is your vehicle and what engine and transmission does it have?