The reason we have it set up this way is because 90% of the time you don't need the time of your fuel-up. Most of the time folks only fill up one time in a day, so the time isn't critical. So we just leave the time out of it so people don't feel like they have to think about it or worry about it. It's just extra stuff to fill out.
However, you do need the time if you're on a road trip and you're driving all day. Then it's important to get the fuel-ups in the right order. So we have the time available there on edit if you need to enter it.
If you use Fuelly Mobile at m.fuelly.com or fuel-ups via SMS, we automatically set the fuel-up time to the current time. We assume you're out and about if you use one of those methods. So that's an option if you're entering your fuel-up pretty close the time you fueled up.
Another option for you is importing your fuel-ups. You can enter your fuel-ups into a CSV text file and you can bypass the form entirely.
I'm sorry you're frustrated by this. People are going to have different needs from the site and we can't make it easy for everyone. We can't make everything a preference either—that has its own set of issues. We need to be able to make decisions about how the site is set up for a majority of users. We know it's not going to work for everyone.