I am writing a term paper on gas mileage comparing 1999 vehicles to 2009 vehicles. Can I have a two sided hypothesis like this one? - With the higher rise in fuel costs over the last 10 years, I am guessing that the average fuel mileage of the vehicles would increase, as entires tune the engines more. Though on the other hand with the rise in safety standard vehicles now carry more weight because of extra air bags, ect. Which could result in a lower miles per gallon.