I recently took an annual trip to Canada, and found myself having to fuel up within the Great White North. I was disappointed to find the website did not allow for individual fill-ups in liters as opposed to my normal gallons.
I guess the easiest way to do this is to... OK, looking at the receipt, this is shot to hell. I paid in CAD. I'm not going to do THAT many conversions for one fill-up.
I wish Fuelly was more multi-dimensional in their offerings.
While it'd be nice to have the units automatically converted, I find it perfectly fine just to manually convert liters purchased to gallons and put in the total price that I was charged by my credit card company (in US$), and letting Fuelly calculate the price per gallon. That way my U.S. and Canada fill-ups both stay in U.S. units. I could see it being a lot of extra effort if I crossed the border more often though (and I'm only 30 miles from it).