I don't think that "(no idea what the margin is)" is what OP meant by running the numbers.
Mostly a "pamper early adopters with VC money" operation I think, possibly with a profitability story (that may or may not turn out to be realistic) centered around high real estate value areas: in those places where the ground occupied by a gas pump and its accompanying infrastructure plays a significant role in the local fuel price, using public rights of way for mobile pumps is effectively a value transfer from the public to the pump operator.
Back during the original dotcom, when all anybody could think of was basically a mailorder shop with a website, I joked with friends how a "fuel.com" could be a great parody of contemporary startups and/or a fine participant in the hype wave.
I still think that sending pickup trucks to fill individual cars is stupid, but it's leaps and bounds less stupid than "mailorder fuel" would have been. A nice illustration of the difference between the current breed of VC money sinks and the dotcoms of old.