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We’ve finally reached the point where the power gets to the wheels. For those of us driving gas or diesel vehicles, only 13-20% of the energy you put in has made it this far.

These forces WILL take up the rest of your power, because you have to overcome them to move forward. How you manage and even reduce these forces will mean using more or less energy.



Aerodynamic Resistance

Portion of Overall Losses: 3% city, 11% highway.
Portion of wheel energy taken: 23% city, 55% highway.

Aerodynamic losses, aka “wind resistance” is the reason so many “hypermilers” go slow on the highway. In the city, at low speeds, your car doesn’t have to push very hard against the air to move. Going faster increases aerodynamic losses, while going slower reduces the losses.

(source: solarjourneyusa.com)

Looking at power requirements, we see that aerodynamic drag quickly becomes the biggest energy eater at speeds above 55 MPH, even for a car with excellent aerodynamics. In fact, by 80 MPH, your aerodynamic drag takes at least double the energy it did at 55, and this is only worse for cars and trucks with bad aerodynamics.

The first thing you can do to improve this is slow down, but as discussed in a previous lesson, don’t slow down too much. Safety is still more important.

Another thing you can do is avoid using accessories that increase your car’s drag. Some things to avoid:

  • Large front license plate mounts
  • Most aftermarket bumpers and “body kits”
  • Metal bumpers, grille guards, etc
  • Roof racks, rooftop cargo bags (put extra cargo behind your car, not on top)
  • Large aftermarket lights
  • Mirror extensions/extenders
  • Large antennas for CB or Amateur Radio
  • Anything else that interrupts the lines of the car

Finally, be sure to drive with the windows rolled up on the highway. Rolled down windows increase drag and lower your mileage. Likewise, close the top on convertibles when going down the highway.

Rolling Resistance

Portion of Overall Losses: 4% city, 7% highway.
Portion of wheel energy taken: 31% city, 35% highway.

Rolling resistance is your tires’ resistance to moving down the road. This varies from car to car. Some tires resist more than others, and even the same tire can resist more or less depending on how loaded the car is, inflation, and weather conditions.

Properly inflated tires will have lower rolling resistance, and when you go to buy tires you can buy special tires with lower rolling resistance. However, when buying a low resistance tire, keep in mind that the tire may have less traction and therefore be less safe. Research low resistance tires carefully before buying.

About the only thing you can do while driving to affect rolling resistance is to keep to smoother surfaces. Paved roads should give less resistance than dirt or gravel roads, and well maintained roads will give better mileage than torn up roads.

Finally, you can practice “ridge riding”. When going down the road, avoid driving in the same exact part of the lane as most other drivers, especially when you can see “ruts” in the road. By going to the left or right a bit, you can keep your tires away from the most worn parts of the road and get improved mileage.

Braking Losses

Portion of overall losses: 6% city, 2% highway.
Portion of wheel energy taken: 46% city, 10% highway.

The First Law of Thermodynamics, aka the Law of Conservation of Energy, tells us that energy cannot be created or destroyed. It can only be converted into different types of energy. There’s no denying that a moving vehicle contains a LOT of energy….so what happens to that energy when you hit the brakes?

In short, it turns into heat. When your brakes slow the car down, they take the energy you put into the car and basically throw it away. The brakes get piping hot, and heat from the brakes dissipates into the air. From there, it floats away into the atmosphere, never to be used again by you. Plus, the more you brake, the more you wear out your brake parts. It’s a lose-lose situation.

Even more tragically, you wasted a lot of energy just to get the car moving and then throw that energy away. If you have a gas motor, it took three quarters of the energy and turned it into heat. That heat was expelled from the radiator and the exhaust, and thrown away, gone. Your drivetrain then took another few percent, leaving giving only 13-20% to the wheels. Rolling resistance then took a cut. Wind resistance took a cut. Now, you’re left with just a few percent of the energy you bought at the pump in the form of your car’s movement.

To save gas, do your best to avoid braking (safely!).

Nobody can avoid all braking. You’d be smashing your car all over the place if it didn’t have brakes. But, you can minimize your use of the brakes by being smart. Several upcoming lessons teach the techniques needed to minimize braking or replace it with something better.

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