Here's some data from a chart a Cessna 152 manual(Lycoming 108BHP). It gives you an idea of how much power is lost as you go up in altitude. Granted, this data is for an airplane, but general aviation is probably the only place you'd find engine data like this. The engine isn't injected, but the table assumes the pilot is adjusting the mixture to ideal lean.

        Alt     RPM     %BHP    GPH True Airspeed
      2000'     2400     77     6.3     102 knots
      4000'     2400     74     6.0     101
      6000'     2400     70     5.8     101
      8000'     2400     67     5.5     100
     10000'     2400     63     5.3     99
     12000'     2400     60     5.0     98

Notice how as you go up, power and fuel usage go down, but you can still maintain the same speed.

-d

At 09:27 PM 10/28/99 -0700, you wrote:
>We live at about 4500', and when we drive to Tucson, at about 2500', we see
>a BIG increase in power.  Our gas mileage is always awful (12 to 13), but
>the Van runs great.  Nothing I've tried has helped.