Backstory: I almost always stop at the same gas station out of convenience. I'm sure there's others around that are cheaper but I also wait until the last minute so I don't risk driving around on empty to find a better deal.
The Idea: A mobile app that would suggest when and where to fill up your gas.
Deep Dive: You would build a mobile app that would connect to your ODB2 sensor and a gas price feed to find the nearest, cheapest gas when you are low. You would refine the algorithm to give the most convenient gas station on the users driving route.
Validation: There are already sites where people post gas prices to find the lowest place.
Monetization: You would have to run ads on this one, the whole point is saving people money.