Startup Idea: Gas Saving Mobile Application

Startup idea to develop an iPhone app that helps users save on gas. The app assesses the cost of gas for each drive and provides comparisons with alternative transportation modes like walking or public transit. It offers a history of the drives and the associated fuel costs for more informed decisions on transportation.

I'd like an iPhone app that would help me save gas. I sometimes drive to places that I could walk to in good weather. How much do those drives cost me? Or if I could take Metro for $2.50, is that cheaper than I would pay in gas to drive (I already can account for parking)? I can figure out MPG, but I feel like that doesn't account for my time sitting in the car and I don't have a good sense for how much each drive costs. I'd love an app that has a history of my drives and the cost I paid in fuel for each one. It would definitely help me make more informed decisions.