I think you mean "how inelastic the demand for gas is in the face of higher prices".
Of course people's income is fixed in this equation. Though I suppose you might try to earn more if gas prices go up.
What you're trying to say is that demand for gasoline is inelastic in the face of higher prices. http://en.wikipedia.org/wiki/Price_elasticity_of_demand
And here's a bit on gasoline... http://economics.about.com/od/priceelasticityofdemand/a/gaso...