This app would be available for mobile devices as well as online. When shopping for a car, it would provide features similar to the existing side by side comparison at fueleconomy.gov - however, it would in addition provide a ROI calculator that would allow you to enter a perspective purchase price for each vehicle (along with the other data the customize function at the above website allows) and the app would tell you (in addition to what the above website currently tells you) how long it would take for the investment in the more efficient model to pay for itself.
The app could be extended to support appliances, using data from EnergyStar.gov's energy guides. The app would take customization information (prices, energy rates, usage patterns, similar to the fueleconomy.gov comparisons) on 2 or more appliance models, and provide a prospective ROI calculation.
Combining these features into one app would make it far more likely that a user will download and install the app on their device, and thus have it with them when they are shopping for a new car or appliance.
The Energy Guides could be improved by adding a QR code that could be scanned to allow easily adding the appliances or cars you are looking at to the comparison app.