Thanks for the suggestion—this is something we're thinking about.
One reason we haven't gone with it is that we'd like to keep the fuel-up form as simple as possible. So for as long as the site has existed, we've asked people to enter the cost per unit.
Once we add another option it complicates that message. Folks will either need to select their preference in their profile, or we'll need to complicate the fuel-up form with another option. That will lead to misunderstandings and confusion. People will either forget which one they have selected, put the wrong numbers in the wrong box, or the right numbers in the wrong box. We have other input methods like Fuelly Mobile and SMS where simplicity is even more key. So instead of having people worry about/remember which option they're using we made the choice to select one and stick with it.
We're not against living with this complication eventually. I think it might be a nice feature and worth this pain of implementing and instructing people. I just wanted to let you know our thinking behind selecting a single option.
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