FUEL EFFICIENCY
Improving Your Mileage

Understand your Engine Understand your Vehicle Understand your Mileage Measuring your Efforts Cost Effectiveness
The Four Cycle Engine Gears & Wheels Mileage vs Gear Vavg vs mpg miles / $  (mp$)
A Four Cylinder Engine Force at the Wheel per gear Acceleration vs Mileage Upper & Lower Bounds THEN
(20 years ago)
The Power Curve   Hyper-Mileage   NOW
(2014)
        Then vs Now
        CAUSE
        SOLUTION

Understand your Mileage

Mileage versus Gear
The chart to the right depicts the mileage (miles per gallon of fuel, or mpg) possible in each gear, under normal acceleration.

The worst mileage is in 1st gear (max power to the wheel). 

Mileage gets progressively better with each up-shift to a higher gear.

Best mileage is in top gear (5th gear in this chart).

Acceleration versus Mileage
Another important factor that affects mileage is how heavy your foot is on the accelerator (the gas pedal.)  The more you depress the accelerator, the more power the engine produces, and the more fuel it uses.  We look at accelerator use in the next three panels, below. 
Maximum Acceleration

The blue dots at the intersection of each torque curve with the next higher gear is where the shift point would be for maximum power and acceleration.

In an automatic transmission vehicle, the driver would floor the gas pedal and keep it there.  In a manual transmission vehicle, the driver would shift gears at red-line.

Unfortunately, shifting gears at the maximum point is also the worst possible fuel economy.  It is also very hard on the engine -- so hard that race car drivers often choose to rebuild their engines after every race,   
 
Moderate Acceleration

This graph, to the right, displays the same possible torque curves for each gear, but we have moved the shift point to the maximum torque point for each gear. 

The blue lines represent what portions of each torque curve would be utilized, and the blue dots and vertical dotted lines represent the shift points.

This would produce a brisk acceleration to speed, and a much improved fuel efficiency over the previous pedal-to-the-metal race car style.  

When you are driving in traffic, especially commuter traffic, other drivers expect you to keep your speed up, or they tend to go into road rage.  So a polite driver is more or less forced to drive at a brisk rate.
Maximum Fuel Efficiency

This graph, to the right, displays the same possible torque curves for each gear, but we have moved the shift point further to the left of maximum torque point for each gear. 

The red lines represent what portions of each torque curve would be utilized, and the red dots  and vertical dotted lines represent the shift points.

The strategy here is to minimize fuel usage by shifting up to the next gear as soon a possible (without stalling the engine.)  And to stay in top gear as long as possible before having to shift to a lower gear. 

Unfortunately, up-shifting at the soonest point can also produces an anemic acceleration to speed.

Hyper-mileage

Done in the extreme, such as often done by  extreme hyper-milers, such slow acceleration as done under Maximum Fuel Efficiency, above, can result in road rage from other drivers caught behind him (or her), as this type driver takes an extremely long time to accelerate to normal traffic speed. 

When you are in normal traffic, accelerate more briskly to flow with the traffic.  But when you can, take your time accelerating.

However, in congested "Stop & Go" traffic, you are often forced to just poke along, with the long line of impeded traffic contracting and expanding by cars speeding up then being forced to slow down, often forced to stop intermittently.   In that situation, it is better to pace yourself, with very mild accelerations when the opportunity arises to speed up a little, and try to avoid speeding up and slowing down.

Selecting routes which allows you to avoid traffic lights and stop signs whenever possible also is a good strategy -- it helps you avoid having to drive in lower gears (with higher fuel consumption rates.) 

Generally speaking, you will get much better fuel efficiency (mileage) on highways than driving in the city traffic, even at elevated highway speeds.
 

2014, Simon R. Mouer III, PhD, PE 
All rights reserved.