Cost2Drive estimates fuel cost for road trips
Punch in your start and end points and what model car you plan on taking and Cost2Drive will estimate how much you can expect to spend on fuel.

Recently, I took a road trip from San Francisco to Los Angeles to drive the 2012 Toyota Camry Hybrid. When I told friends that I'd driven over 5 hours one-way just spend even more time driving when I got there, the question I got most often was, "Wouldn't it have been cheaper to just fly?"
Had I known about the Cost2Drive Web service and iOS app at the time, I'd have been better equipped to answer that query.
After entering a starting address, ending address, and a make, model, and year of a vehicle, Cost2Drive will estimate how much one should expect to spend fueling a road trip. The service calculates this cost based on the distance between the points, the highway fuel economy estimate for the selected car, the size of the car's gas tank and what type of fuel it uses, and fuel prices along the route.
The service will also show where and how often to stop for the lowest prices along the way. So, you can see that although trip costs for a
Cost2Drive's estimates are given only for one-way trips. The option to generate round trip estimates would be nice, but I suppose one could just double the one-way estimate in most cases. In addition to the total fuel price, Cost2Drive displays the distance, driving time, and carbon footprint for the selected trip. There's also an option to purchase carbon offsets if you'd like to greenwash your trip.
The service also rolls in the Kayak trip planning service to display the lowest airfare to the chosen destination to show how much more or less you'd spend if you just skipped the whole driving rigmarole and flew instead. For those wondering, Cost2Drive estimated that I'd spend $36.89 on regular gasoline if I made the trip from San Francisco to LA again today in a 2012 Honda Civic. A flight from SFO to LAX would cost me $119 (and I'd still have to rent a car when I got there). So, it looks like I made the right choice when planning my trip.
Cost2Drive is a free Web service. There's also an iOS app that's available for $1.99 in the iTunes App Store.