Altitude has a big effect on the fuelling of the pumps as the air density and pressure are of the greatest importance. Higher up, air density decreases and the amount of oxygen in the air diminishes as altitude increases. The required air-fuel mixture for proper combustion is specific, and any deviation in this ratio has a direct effect on an engine's power output. For instance, in altitudes above 5,000 feet, engines can lose up to 20 percent of their power versus power at sea level, which can require adjustments to the fuel delivery to operate correctly.
We read a lot about how a fuel pump works, especially with an electronic fuel injection (EFI) system: sensors will respond to height changes to crank up or reduce the amount of fuel pressure and quantity. Many of these systems will automatically modify the air-to-fuel ratio, but with older mechanical (or just less than fully functional—experienced enough, hotrods, and you know what I mean) fuel systems, there's no compensator for the decreased density of the air going into the cylinder, and thus the resultant combustion runs very poorly. Some cars and trucks running on carburettors, for instance loose specifically 15-25% of fuel efficiency at high altitudes that owners can fix by changing jets or adjusting the mixture to keep their cars working how they should.
Pump performance with regards to maintaining pressure levels is also challenged at altitude. In places such as Denver, Colorado rises to 5,280 feet, which means fuel pumps work to maintain normal sea-level pressure levels (30 to 80 psi, depending on the vehicle). This creates stress that can shorten the actual service life of the pump, resulting in accelerated wear or even overheating in some instances. Ford and other car manufacturers go through serious testing for high-altitude work to optimize the fuel system in vehicles that will be sold in high-altitude areas, where the importance of keeping the fuel pump efficient to keep the engine running can be crucial.
Hitting on this point is Gale Banks, founder of Banks Power and automotive expert, who explained that "Performance engines lose power with each passing thousand feet of elevation gained." This is why adjustments are needed in fuel systems to combat these effects." This highlights the significance of an effective Fuel Pump especially when driven at high-adjustment conditions, for this can limit vehicle performance and vehicle economy even in nominal values.