Is there a way to add multiple drivers for a car to compare driving habits?
Yes, you can share a car with another driver so that they can add fuel-ups as well. But there's no feature that compares the two different drivers in any way. One thing you could do is add a note to each fuel-up indicating who added it, and you use that to help you remember who did what, when. But the only information Fuelly is tracking is fuel-ups. There's no indication who was driving during a particular time period so I'm not sure a feature looking at two different people who added fuel to a vehicle would tell you much.