Car's Max. Fuel-Efficient Speed

[Geremia]

What is the thermodynamical reason that a car's maximum fuel efficiency is for speeds 55-60 mph? (See http://www.mpgforspeed.com/ )

 

[Bob S]

In your link, the maximum fuel efficiency curve shows a nearly flat region between 35 and 55 miles per hour. There are many contributions to the curve.
1) Air drag force is proportional to velocity squared. the power scales as velocity cubed.
Air drag depends on the frontal area of the vehicle, the drag coefficient, and the air temperature (and elevation).
2) The engine is most efficient at about 80% of max torque, and 35% of redline.
3) all other major losses (e.g., tire rolling friction coefficient) are linearly proportional to velocity.
4) For automatic transmissions, subtract 15% from your mpg.
5) If you have an automatic w/ torque converter lockup, use it.
So, derive the equation for air drag.

 

[astrokreng]

The thermodynamic reason for a car's maximum fuel efficiency being at speeds of 55-60 mph can be explained by the principles of aerodynamics and engine efficiency. At lower speeds, air resistance is not a significant factor in the overall energy required to move the car. As the car speeds up, however, air resistance becomes increasingly important and requires more energy to overcome.

At speeds above 60 mph, the car experiences more drag due to increased air resistance. This means that the engine has to work harder to maintain the same speed, resulting in a decrease in fuel efficiency. On the other hand, at speeds below 55 mph, the engine is not operating at its optimal efficiency, leading to a higher fuel consumption rate.

Additionally, the design of the car's engine and transmission system are also optimized for speeds of 55-60 mph. These components are designed to operate most efficiently at this speed range, ensuring that the car's fuel consumption is minimized.

In summary, the thermodynamic reason for a car's maximum fuel efficiency being at speeds of 55-60 mph is a combination of reduced air resistance and optimal engine and transmission design. Any deviation from this speed range will result in a decrease in fuel efficiency due to increased drag and suboptimal engine performance.