At what speed can I achieve the highest fuel efficiency?

This question is more complex than it seems. You're essentially inquiring about the ideal constant speed for optimal fuel economy, excluding factors like stops and starts. Let's assume you're on a lengthy highway journey and want to maximize mileage. We'll begin by examining the power required to propel the car forward.

The power needed to move a car forward changes depending on its speed. This required power can be calculated using an equation of the following form:

road load power = av + bv² + cv³

In this equation, v stands for the car's velocity, while a, b, and c are distinct constants:

• The a factor primarily stems from tire rolling resistance and internal friction, such as brake pad drag or wheel bearing friction.
• The b factor also includes component friction and tire rolling resistance, along with the energy consumed by the car's various pumps.
• The c factor is largely influenced by aerodynamic drag, determined by factors like frontal area, drag coefficient, and air density.

These constants vary for each vehicle. However, the key takeaway is that doubling your speed significantly increases the power needed—far more than just doubling it. For instance, a medium-sized SUV that uses 20 horsepower at 50 mph could require 100 horsepower at 100 mph.

From the equation, it's clear that when the velocity v is 0, the power needed is also 0. At very low speeds, the power requirement remains minimal. This might lead you to believe that driving at an extremely slow speed, like 1 mph, would yield the best fuel efficiency.

However, the engine operates in a way that debunks this idea. Even at 0 mph, the engine continues to run, consuming fuel to keep the cylinders, fans, pumps, and generators operational. Additionally, the use of accessories like headlights and air conditioning further increases fuel consumption.

Thus, even when stationary, a car consumes a significant amount of fuel. Fuel efficiency is at its worst at 0 mph since the car burns gasoline without covering any distance. When the car starts moving at 1 mph, the fuel usage increases only slightly due to the minimal road load at this speed. At this pace, the car travels 1 mile in an hour, drastically improving mileage. Doubling the speed to 2 mph results in a small increase in fuel consumption but doubles the distance traveled, nearly doubling the mileage.

Efficiency of an Engine

Essentially, the engine's efficiency improves as speed increases. A fixed amount of fuel is used to power the engine and accessories, while a variable amount is consumed based on the power needed to maintain speed. Therefore, in terms of fuel used per mile, higher speeds make better use of the fixed fuel required.

This pattern persists up to a certain point. Eventually, the road load curve overtakes the efficiency gains. Once speeds reach around 40 mph, each additional mph demands a substantial rise in power. Beyond this, the power needed escalates faster than the engine's efficiency improves, causing mileage to decline. Let's apply some speeds to our equation to compare a 1 mph increase from 2 to 3 mph with one from 50 to 51 mph. For simplicity, we'll assume a, b, and c are all set to 1.

It's evident that the power increase needed to accelerate from 50 to 51 mph is far greater than that required to go from 2 to 3 mph.

Thus, for most vehicles, the optimal speed range, or "sweet spot," lies between 40-60 mph. Cars with higher road loads will achieve this sweet spot at lower speeds. Key factors influencing a car's road load include:

• Coefficient of drag. This measures a car's aerodynamic efficiency based solely on its shape. Today's most aerodynamic cars have a drag coefficient roughly half that of many pickups and SUVs.
• Frontal area. This is primarily determined by the car's size. Large SUVs can have more than double the frontal area of smaller cars.
• Weight. This impacts the drag exerted by the tires. Large SUVs can weigh two to three times as much as the smallest cars.

Generally, smaller, lighter, and more aerodynamic cars achieve their best mileage at higher speeds. Conversely, larger, heavier, and less aerodynamic vehicles perform best at lower speeds.

Driving within the optimal speed range, or "sweet spot," ensures the highest possible fuel efficiency for your vehicle. Deviating from this range, whether by going faster or slower, reduces mileage. However, the closer you stay to this ideal speed, the better your fuel economy will be.