Fuel injected vehicles have a computer that adjusts the amount of gas to match the oxygen in the air. That means even if the vehicle is operating at high altitudes where the air is less dense and thus has less oxygen per volume the computer will compensate with less fuel. So operating a fuel injected engine at altitude should not lead to a decrease in fuel economy through over rich mixtures. However, the reduced air density will decrease air resistance, especially at highway speeds. So if you are operating a fuel injected car at city speeds at altitude, you will probably experience little change in the gas mileage. If you are doing long highway speed cruises, you quite probably will see an improvement in your fuel economy.