Suppose that the price of gasoline has risen by 50%. What happens to a consumer's level of ...

Initially, the consumer is on budget constraint BC1, consuming g1 units of gasoline on indifference curve I1, where M is the individual's income level and P1 is the price of gasoline.  If only the price of gasoline changes to P2, the horizontal axis intercept of the budget constraint moves towards the origin.  This is illustrated above by a movement to the budget constraint BC2.  On indifference I2, his level of satisfaction is lower than before.    

Now, if the individual's income increases just enough so that his original consumption bundle exactly exhausts his new budget.  However, the slope of the budget constraint (BC3) that runs through his original consumption bundle is steeper due to the higher price of gas.  This also implies that his MRS is less than the ratio of prices.  Thus, the individual can attain a higher level of utility by purchasing less gasoline than g1.  The individual is better-off at higher prices and income than at original levels.