The Bullet and the Foot, Nov 10, 2009

Quote of the Day
Kindness is a language which the deaf can hear and the blind can read.
		-- Mark Twain
Whoops, slipped another day
Well I guess one benefit of changing the column's name from "The Daily Geek" to "The Bullet and the Foot" is that "Daily" is no longer implied. Still, I feel like I forgot to shower when I go a day without dropping in a new article. So my apologies, dear reader for my absence yesterday.
Zen at the Art of Aggressive Driving
With a big driving holiday coming up, I figured I'd start a few columns over the next few weeks on the Zen of driving. And, as it turns out, there is really only one way to survive the experience, so I present the perversion that is "Zen and the Art of Aggressive Driving",

For our first chapter, I want to ingrain into you all that the lessons from your Physics class you ignored in high school can save you money in the least, save your life at the most. And the biggest cause of accidents, and the greatest cause of fuel inefficiency is not speed. It's changing speed.

A quick raise of hands, who here remembers Newton's laws of motion? Okay, well I know you have your hand up, but the rest of the class is looking a bit sheepish, so here is a quick review for their benefit. Actually, let's just cut to the physics of cars.

The first two are particularly useful here. The first, roughly translated from 17th century geek, is that something at speed will remain there unless acted upon by a force. In your car, there are 3 basic forces that affect speed:

  • The accellerator converts fuel into the energy of motion
  • Friction converts the energy of motion into heat (thus slowing you down.) And your brakes, BTW are simply a controlled form of friction.
  • Collisions transform the motion of energy into changes in shape of both the car and the driver.

This should take the mystery out of the physics of driving. Think of the speed of your car as a bank account. Applying gas deposits money in the account. Applying brake is a withdrawal. The speed your car is at is the balance between the deposits and the withdrawals.

When you are at highway speed, and you take your foot off the gas, your car will keep moving on it's own. It slows down slightly because of rolling friction and air resistance, but by in large if you aren't changing speed, you don't need to apply a lot of gas. This is why "Highway" rating for mileage are higher than the "City" ratings.

The people who get shitty mileage are the ones who think the car has two states: gas on, or brake on. You've seen them on the highway. You may be one for all I know. But the tell-tale sign on one who doesn't grasp energy is one who is constantly chewing through brakes. They don't trust that the car will maintain speed, so they are constantly accelerating, until they have to brake.

I'm pretty sure you know who I'm talking about. That SUV who races ahead, to sit on a bumper. Then changes lanes, to get ahead. And when he or she is finally in out in front in the left lane, they suddenly remember there is a speed limit, and park themselves in someone's blind spot of the guy in the middle lane with his cruise control on.

Regardless of where the cruise control is set to, having it on is your best bet for highway economy. Don't get me wrong, I find the hyper-milers extremely annoying too. They misapply the concept of wind resistance, and ignore the reality that the loss to friction is more than offset by operating your engine at it's most efficient RPM range. Plus, when you are driving at 50 on a 65mph road, you really look like an asshole.

The optimum RPM is something that varies from car to car. My stick shift 2005 Subaru Imprezza operates at it's peak somewhere between 70mph and 75mph, depending on whether I have a headwind or a tailwind. And yes, I do track my milage. I average 29mpg. The EPA estimates I should only be getting 27. The EPA rates the car at 55mph.

Much faster that 75, and my mileage does tail off quickly though. Plus speeding tickets tend to get downright criminal above 80. So I tend to operate on the 70-75 range on 65mph roads, and 65-70 on 55mph roads, and exactly the speed limit in work zones. (Economy be damned, when a roadway worker's safety is at stake.)

You can tell when your car is in the butter zone, not by the speedometer, but by how it reacts to the gas pedal. If you have your foot to the floor, and going flat-out, you are not operating your engine efficiently. It's when you move the pedal slightly down, and the car feels like it wants to leap ahead. That's your optimal rpm range.

I could ramble on for days, so let me wrap up today's, and tomorrow will be tips on city driving.

All content copyright 2018, Sean Woods | email: | phone: 703-342-2662