The Effect of Elevation on Gas Mileageby Tim Anderson
A common question for drivers who have traveled in their vehicle from a lower altitude to a higher one or vice versa is why there is a difference in gas mileage between the various altitudes. And while the overall explanation might seem overly scientific at first glance, the effects of elevation on your gas mileage are actually fairly rudimentary.
At its most basic level, octane is nothing more than a rating given to gasoline to describe its “anti-knock” performance. That is, its ability to resist uncontrolled combustion within the cylinders of an engine. Gas is supposed to be ignited by the spark plugs, but when it ignites from compression it causes a “knocking,” which is an explosion of air and gas that can damage the engine. Lower octane levels combust from compression at lower levels of pressure.
Importance of Octane
Octane levels are important because they help prevent these explosions from happening. Over time, prolonged knocking—more commonly referred to as pinging—can actually cause damage to an engine and even result in power loss over time. Different engines are built with different octanes in mind, and your user’s manual will let you know which octane you are supposed to use in your vehicle.
Engines work by compression, and the compression ratio of your engine determines the level of octane required by your particular car. By increasing the compression ratio of an engine you can actually increase the horsepower of a vehicle, although it will burn more fuel this way. By lowering the compression ratio you lower its output, but also increase the fuel efficiency.
Effects of Air Pressure
Altitude decreases an engine’s octane requirements because of the change in air pressure. The higher elevations have a lower level of air pressure, which means an engine needs less octane to properly fire due to the lower ambient pressure. In certain states, such as Colorado where the altitude is generally above 5,000 feet in the mountain regions, 85 octane gasoline is sold, while in lower elevations 87 octane is the lowest sold.
When an engine requires lower levels of octane to “fire,” the overall efficiency of the engine is also lowered, which means the engine works harder to achieve the same results. However, less fuel burns, which means the overall fuel-per-mile efficiency is increased because the engine is working at a reduced compression rate. In short, you can get higher miles-per-gallon at higher elevations, but the power of the engine is also somewhat reduced.
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