I know this blog is about greening my house, but the fact remains that one of the largest portions of the green house gas emissions that we produce are due to our cars. This makes sense when you see how much energy it really takes to get a car moving and compare that to the energy it takes to light a light bulb.
So, anything we can do to save gas saves a lot of energy, green house gasses, and oh yeah, let’s not forget money!
So what is this blog entry about? Hypermiling. Simply put, hypermiling means trying to drive your car is such a way that you stretch your gas as long as it can go. It sounds a little far-fetched, but I’m telling you, it can work! There are a few hard-core crazy people that have gotten Toyota Priuses to drive over 100 miles per gallon without any mechanical modifications to the car. The way they do it is with a few crazy techniques that most people will not want to do. The rest of us will never achieve this sort of mileage. But, with a few simple tricks, you too can increase your mpgs a significant amount and save money while doing it.
Some of the “extreme” techniques of hypermiling are dangerous, so I have avoided them. I’d rather waste gas than compromise my own safety.
Also, it is very useful to have an instant-read mpg meter in your car, so you can tell what effect your behaviour has. After playing with the settings, I discovered that our bimmer actually has this built right in already. The Prius of course has it as well in the great big center display.
So without further ado, here are the “tricks”:
Tire Inflation
One of the ways that cars experience drag is through the tires. If the tires are sticky, or very warm (like in the desert), or if they are underinflated, they will give more drag (called rolling resistance) than normal. So one way to increase your mileage is to make sure they are properly inflated. I have started checking once a month or so when I am at a gas station with an air pump, and I’m not in a hurry to get anywhere. The tires have the recommended inflation written right on the side walls. I am keeping my tires near, but not at the top, of that recommended range. This is a safe and easy way to increase your mpgs.
Coasting
Warning! Before you go and try this technique, check your state laws. Apparently, it may be illegal in some places.
The idea here is that you put your car in neutral while decelerating so that you use all the kinetic energy stored in your momentum to its fullest advantage. When you apply the brakes, this kinetic energy from the motion of the car is transformed into heat energy in your brakes, which is then dissipated by the air rushing by. That heat represents inefficiency and waste.
I coast for example when exiting a highway. I anticipate the exit and coast to the surface road speeds in neutral and then put it back into “drive” when I have decelerated to the proper speed. I have gotten especially good at estimating when to put it in neutral for my home and work exits such that I reach the proper city road speeds by the time I finish getting off the exit ramp.
You can also use this technique driving on city roads. I try to look far ahead to the next stoplight and see if it is red. If so, I immediately start coasting so that there is less time spent braking. If I am just accelerating from a stop and I see that the next light is red, I don’t accelerate to a normal speed. Instead, I coast in hopes that the light will turn green before I get there.
For those areas where coasting in neutral is illegal, you can coast in “drive” too. Simply let off the gas and let the car slow down by itself. It won’t coast as far as it would in neutral, but it will save some gas.
Consistent Speed
Acceleration uses up extra gas. You can imagine that if it takes a certain amount of gas to overcome ground and wind friction and keep you going a certain speed, then it will take even more gas than that to actually accelerate you. Similarly, braking represents shedding excess energy that you should not have spent earlier accelerating in the first place. So, the most efficient way to travel is to avoid acceleration and deceleration as much as possible.
Unfortunately, acceleration is unavoidable, as you need to actually start moving in order to get anywhere. However, there are a few things you can do to mitigate it.
One important thing to do is to avoid jack-rabbit starts. That is, avoid accelerating very quickly from a dead stop like a jack-rabbit might. The reason for this is that engines operate with a power curve. That is, at lower rpms, the engine produces lower power and the power per volume of gas is lower. As the engine accelerates in rpms, it produces more power until it reaches a peak, at which point it starts losing power as more and more gas is wasted turning the engine slightly faster. In order to increase the rpms at low rpms and very high rpms, you have to waste a lot of gas. Somewhere in the middle of the rpm range is the optimal distance per gallon ratio. This is different for each engine of course.
So, this means accelerate more slowly when starting from zero. If you accelerate too slowly, then you are spending a long time in the low-efficiency rpm zone of your engine. So, you have to strike a balance. Somewhere in the middle, you can accelerate with the least amount of gas.
On highways, the idea is to drive your car at or near optimal speed and stay there. For most cars in the US, the engines are tuned to produce the maximum efficiency right near or under 65 mph. So, if you go over that speed, you are wasting gas overcoming wind and rolling resistance. Not to mention, you are risking a speeding ticket on most highways!
If you drive a consistent 60 to 65 in the right-most or 2nd to right-most lane, you will not annoy anyone, and you will save gas. Often, a cruise control will do a better job than humans will at keeping a consistent speed and rpms, so I use it whenever the traffic is light enough that I can drive for a long time without disengaging it.
Let Off the Gas
For many automatic transmission cars, if you let off the gas a slight bit while driving at a constant speed, the car does not actually slow down very much or at all. However, the mpg goes up pretty nicely! I see this phenomenon in both the bimmer and the Prius. Having the instant read mpg meter really helps to see this one.
The reason this happens is that at any given point the automatic transmission is set at an estimate of the optimum ratio for the current speed and intentions of the driver. However, this estimate is sometime off. If you are cruising at a constant speed, you can “help it out” by letting off the gas just slightly and putting the transmission in the most efficient mode.
My bimmer goes from the mid 20s to the mid 30s when I do this. I’m not sure that it is really getting mid 30s, but at least it is an increase.
Draughting
The crazy “extreme” hypermilers draught very close behind the large 18 wheelers. This creates significant mpg savings because the truck is essentially “cutting the wind” for you. The truck creates a slight underpressure behind itself when moving through the air, and if you drive in that underpressure, it will means you have less pressure in front of you — ie. less wind resistance.
However, that type of draughting is not only illegal everywhere, it is very, very dangerous, and truckers also hate it when you do that. Basically, you are tailgating, and when the truck stops suddenly, you have very little time to react and slam on the brakes yourself. Crunch! Don’t do this.
But why do I have a section on draughting then, if it is so dangerous?
Well, it turns out that if you drive a normal distance behind a large truck — not tailgating — you will still experience a slight increase in mpgs. I have seen as much as 5 to 10% sometimes with the bimmer. This means improving from 22 to 24 mpg. That doesn’t seem like much, but that can add up to over a hundred bucks a year if you do it right.
So now what I do if I see an 18 wheeler that is driving my speed, I try to slip in behind them and drive a normal, legal distance away.
Another thing you can do is “side draughting”. That is, drive right behind the large truck at the same speed, but one lane over. If they slam on the brakes, you will not slam into their backside because you are in a different lane. The idea is that a truck moving through the air creates a bow wave much like a boat does in the water. If you “surf” this bow wave so that the trough of this wave is in front of your car and the crest of the wave is in the middle or behind your car, you can experience 5 to 10% improvement as well.
Now, side draughting has drawbacks as well. If you are in the wrong spot, you may be in the truck driver’s blind spot. I always make sure I drive in a spot where the truck driver can see me. If the side draughting puts me in a spot where he can’t see me, I don’t do it. Also, it is difficult. Many times, the truck driver is inconsistent and speeds up and slows down and speeds up and slows down. It is difficult to find the exact right spot and maintain it when the truck driver drives like that. Actually, it is difficult to find the exact right spot anyways, even if the truck driver drives consistently. Another problem is that now there are 2 lanes of traffic moving slowly, and you may be holding up all the other crazy drivers. I usually only do side draughting when there is not much traffic.
Reduce the Weight
This one is safe and easy. Basically, all of that stuff you are lugging around in your trunk and back seat — take it out! You don’t need to waste gas hauling it around. Only leave the bare necessities like a set of jumper cables or flares. Put those shoes away in your closet. Put that TV you were going to give to your nephew in the garage until you are ready to give it to him.
I took out a bunch of bags, one of the two umbrellas, a bunch of work out clothes, and a blanket. Probably altogether it was only 5 or 10 pounds of stuff. That doesn’t seem like much but when I calculate how much it cost to haul 10 pounds of stuff for a year, it comes to about 4 gallons of gas, which at today’s prices is about $16, or 80 pounds of CO2. I know many other people are carrying around a lot more junk than I was. (My sister in law for example!) They would save even more gas and money than me.
And, if you can swing it, lose some weight yourself. It will save gas, and you’ll be more healthy yourself! To be honest, I have not been able to achieve much savings in this area, but hey. If it works for jockeys and America’s Cup sailors, it can work for us too. 
Turn Off the Engine at Long Stoplights
You can save a small amount of gas by not idling at a stop light. I often stop my engine if the light it going to be longer than 10 seconds. Mostly I do this for stop lights that I go through frequently and I know they always take a long time.
Why 10 seconds? Well, it turns out that many cars use up more gas starting up than just idling. My car use about 10 seconds worth of gas to start up again, so if I am going to be waiting longer than 10 seconds, I will be saving a small amount of gas by doing that. Each engine is different. Some cars are more efficient and they use only 5 seconds worth, and some cars, especially older ones use far more than that. For those older cars, turning off the engine probably doesn’t make sense at all.
The Prius of course does it automatically for me. When it restarts the engine again, it is using the large electric engine to get it going, so it uses very little fuel in restarting. They have done a pretty good job in making it as close to imperceptible as possible. (I can still tell when it starts though.
Now turning off your car means you have to be able to trust your car. If it doesn’t start easily, don’t do this. Our bimmer runs nicely and starts reliably, so I feel comfortable restarting. Also, I don’t want to hold anyone else up, so I watch the stoplights for the opposing direction. When they turn yellow, I start my car and put it in “drive” so that I am ready to go by the time our light turns green. If I can’t see the opposing direction’s lights, it probably means I am far back in a queue for my own direction. In this case, I usually start the car as soon as the light turns green for us and I am ready to go by the time all the cars in front of me have gotten out of the way.
How Much Has it Saved Us?
Well, this is the big question. Was all of the above worth it? Well, the mileage I was getting on the bimmer I drive to work was 22. Now I am getting between 26 and 28.
Calculating: increase of at least 4 mpg / 22 mpg = 18% increase in mpg
Wow, that’s not bad! Let’s see how much that means in terms of gas.
I drive 23 miles to work, and then the same home again of course. That’s 46 miles.
46 miles * 255 working days a year = 11730 miles a year
11730 miles / 22 mpg = 533 gallons of gas under the old driving habits
11730 miles / 26 mpg = 451 gallons of gas under the new driving habits
Savings: 83 gallons of gas
83 gallons * 20 pounds of CO2 per gallon = 1640 pounds of CO2 avoided
Now we get to the good part. Gas in our part of the country is up around $3.90 a gallon.
83 gallons * $3.90 = $324 saved!
And that’s just from the commute! We actually put on about 20K miles a year on that car. Using that figure, here’s what we get:
Fuel saved: 140 gallons
CO2 saved: 2800 pounds
$$ saved: $546
Now we’re talking!
