Don't obsess over MPG, it's GPM that matters.
It's already been pointed out that diesel mpg can't be directly compared to gasoline because diesel fuel has higher energy and carbon content. But more to the point this obsession with super high MPG numbers is harmful to improving fuel efficiency.
Let's say you have a beast of an SUV that gets 10MPG and drive 10K miles in a year. That's 1000 gallons of fuel to drive that car. Now replace that car with something like a minivan or one of the newer crossover vehicles that gets about 20MPG. Now you use 500 gallons of fuel, a savings of 500 gallons. And unless you have a really large trailer that you 'need' to tow, you've probably given up nothing significant. Go from 20MPG to 30MPG and you use 333 gallons of fuel, saving 167 gallons. Even going from 20MPG to 40MPG you save 250 gallons, still a good savings but you give up pretty significant capability (passenger seating, load and towing capacity. Of course if you don't need to seat 7 and their gear, go for it.) And going from 50MPG to 60MPG? You save 33 gallons of fuel in a year. Not that we should stop trying to improve mileage at any point, but convincing virtually everyone to increase to the maximum mileage vehicle that still meets their needs is a much bigger savings in fuel than getting a handful of specially eco-sensitive folks to drive super high mpg vehicles. (And I know people who've chosen to by a Yukon over a Traverse because the traverse would 'only' get 5MPG better mileage. Best change the EPA could make would be to sticker cars with "gallons per 10K miles" rather than MPG.)
