28 MPG is pretty average for many cars on the road, so 31 MPG is good.
There are many online shops that provide best mpg for auto cars. Amazon.com is one of online shop that provides best mpg for auto cars. You can visit www.amazon.com
There are many efficient cars out on the market today. The best MPG car you can buy at the moment is the 2012 Smart For two Coupe at $347/MPG. Coming in next is the 2012 Nissan Versa Manual at $366/MPG.
21 mpg on average (cars current condition may effect slightly)
vw.
5.22
Right now, the best MPG cars on the market are hybrids. The Ford Fiesta and Nissan Leaf are just two cars that get some of the best MPG right now. These two cars have an incredible MPG and can get anywhere from 70 to 90 MPG. Before purchasing a vehicle research its fuel efficiency to ensure that you will get the most miles to the gallon.
"gas mileage generally goes from 25-28 mpg in the city to 34-40 mpg on the highway" - http://www.cargurus.com/Cars/Overview/-c5816-1995-S-Series.html
Railroad do not get 400 mpg with diesel fuel. They are rated in gallons per mile and there are way too many variables to give a single overall answer.
674 miles @ 21 mpg will require 674/21 = 32.1 gallons (approx).
A good place to compare cars' fuel economy is at http://www.fueleconomy.gov. This includes statistics of smart cars and regular cars to compare them.
http://www.fueleconomy.gov/