A “family” is looking to trade in one of their vehicles to save on gas.  Both vehicles are driven approx the same number of miles per year.  

#1.  One vehicle is a van getting 10 mpg and they are thinking of trading it in on a station wagon that gets 20 mpg, or a difference of 10 mpg.   

#2.  The other vehicle is a sedan that gets 30 mpg, which they are thinking of trading for a hybrid that gets 50 mpg, a whopping difference of 20 mpg.   

Which way should they go to save the most gas? 

Converting #1 to gallons per mile:   .1 gpm vs .05 gpm — a difference of .05 gpm.  Trading would save 2 times the gas.

Converting #2 to gallons per mile:  .033 gpm vs .02 gpm — a difference of .013 gpm.  Trading would save 1.65 times the gas. 

Answer:  trade in the van for the station wagon.   They will use almost 20% less gas by trading in the van, vs. trading in the sedan.