Greg Studen | 05/10/09 @ 9:24pm
Your tachometer is the dial on the dashboard of your car, next to the speedometer, that tells you the engine speed in revolutions per minute (rpm). The tachometer originated for use in performance cars and commercial equipment, such as tractors and trucks, where it is important to know engine speed as well as ground speed. Knowing engine speed in commercial and performance uses is important because torque, or turning power, varies with the speed of the engine, and excessive wear or damage to the engine can be caused at high speeds. A tachometer is used in industrial or farming applications to run equipment at a constant speed.So what's the use of a tachometer in your family car? In a standard transmission car, the tachometer can be used to give a visual guide to when you should up or downshift. When the engine rpm reaches a certain level, usually about 2500-3000 rpm, torque is maximum, and there's no benefit to running the engine any faster. Watching the tachometer can tell you when to upshift. However, most cars today have automatic transmissions--you press the gas pedal and it goes. You never know or worry about what gear you're in or how fast your engine is turning. Tachometers, which are still provided on many cars, seem useless. They give the car dashboard a sporty look, but who bothers to look at how fast the engine is turning?Here's a suggestion for how a tachometer can become a useful tool, whether you have an automatic or standard transmission. And, if you don't have a tachometer, you can still learn how to apply and benefit from the principle behind its use. The goal is to save on gas, and to reduce CO2 emissions into the atmosphere. The basic idea is that the faster your engine is turning, the more gas you burn. So, you use the tachometer to keep your engine rpm's, on average, as low as possible. But before going into the details, let's look at a few numbers that will give us an idea of what's at stake, both in terms of dollar cost and the addition of global warming CO2 gas into the atmosphere.In order to assess how much we might save by monitoring our engine rpm, we need to start by doing a few calculations of how much gasoline we use. Let's assume an average gas mileage of 20 miles per gallon, and annual distance travelled of 10,000 miles. That means 500 gallons of gasoline per car (you can adjust the numbers to fit your situation). Gasoline weighs about 6.2 pounds, so multiplied by 500 that's 3,100 pounds, or one and a half tons, of gasoline per year--as much a the weight of many cars! Each pound of gasoline burned emits about 3.36 pounds of carbon dioxide into the atmosphere, so you get 20.8 pounds per gallon, or about 10,400 pounds, or five tons of CO2 from each car per year, using these assumptions. Now let's see how we can use the tachometer to reduce our gasoline consumption and CO2 emissions. The crucial technique is to push rather gently on the gas pedal as you accelerate. If you have a tachometer, watch it as you speed up from a standing start, and try to keep it at or slightly over about 2000 rpm. You will definitely accelerate more slowly than usual--most of us bear down pretty hard on the gas in order to speed up quickly. Accelerate quickly and watch the tachometer jump up to 3000 or 3500 rpm before the car shifts to a higher gear. With slow acceleration, keeping the tachometer at about 2000 rpm, the automatic transmission will upshift as you reach the appropriate speed; you'll see the tachometer drop down to about 1600 rpm when it does. With a little practice, you can keep the engine speed to 2000 rpm or less pretty easily. Of course, this doesn't mean that you can't accelerate rapidly if you have to, say in heavy traffic or when merging into the freeway. The point is to get your average rpms down. It doesn't matter whether you drive a gas guzzler or a thrifty hybrid; you will get the same proportional reduction in fuel use from keeping those rpms as low as possible.If you don't have a tachometer, you can train your foot to accomplish pretty much the same results. Just think of accelerating very slowly. A fast car, for example, might go from zero to sixty miles per hour in six seconds--that's with the gas pedal pushed to floor and and the engine roaring. Since you're aiming for the opposite effect, try to go from zero to sixty in about twenty seconds, or from zero to thirty-five in about 15 seconds. That should give you the right feel. In any case, if you feel that you're accelerating slowly, you are no doubt doing fine.A few additional tips will keep your average rpms low and save gas. In addition to slow acceleration, you want to strive for slow deceleration; that is, slow breaking. So it's easy on the gas, easy on the breaks. Why slow breaking? Here the objective is to maintain your speed at any given moment as close to your average speed for the trip as possible. By doing this, you avoid the necessity of additional acceleration each time you have slowed down and then have to speed up again. At lights, coast up to the red light if possible, and keep moving if you can as it changes to green. Drive at or close to the speed limit. Your engine speed is directly proportional to your ground speed in any gear. If you can time trips when there's little traffic, it will reduce the amount of stop-and-go, and will save gas. You can also plan trips to avoid lights and left turns, if possible without lengthening the trip too much. The whole idea is to keep it moving, slow and steady. In normal city and suburban traffic, you're going to get where you're going in almost exactly the same amount of time. For those who really want to get into this, there are a whole range of techniques called "hypermiling," aimed at reaching maximum performance. See their website at http://www.hypermiling.com/. Go to the home page and click on "See the hypermiling How-To methods" in the middle of the page. You might also want to take a look at the Wikipedia entry on hypermiling. There's even a Fuel Economy World Championship, held at Elkhart, Indiana, where a Honda Insight (pre-2006 model) holds the record with 213 mpg for a round trip! A Toyota Prius in the same contest got 136 mpg, and a Ford Escape hybrid got 76 mpg. A team of Honda Insight drivers in a separate contest drove a total of 2254 miles on a single tank (13.7 gallons) of gas, for an average of 164.53 mpg. The EPA mileage estimate for the Insight is "only" 60 mpg. As they say, don't try this at home, but these numbers do give a good idea of the phenomenal increases in gas mileage that can be achieved with advanced techniques. Here's the payoff. Let's say that you average 2500 rpms over the entire range of your yearly car use. This includes all your rapid acceleration, driving too fast, and breaking too hard and too often. With a little effort you could reduce that average rpms from 2500 to 2000. Assuming that gas usage is roughly proportional to engine speed, you would then save roughly 20% on your gas usage. Using our previous calculations of yearly gas consumption, that's 20% of 500 gallons, or 100 gallons of gas. That's 620 pounds of gas, and at 3.36 pounds of CO2 per pound of gas, you get 2000 pounds, or one ton, of avoided greenhouse gas emissions. With a little more effort, you could do better.
The real benefit comes from the potential cumulative effect of everyone driving this way. There are about 135,000,000 cars in the US. If we can save one ton per car, that's 135 million tons of avoided CO2 emissions. This is just one example of a way to achieve substantial savings in imported oil, money, and global warming emissions by a rather simple change in behavior--with no real dimunition of "standard of living." The use of the tachometer illustrates a vital general point about the effort to achieve more efficiency in everything we do: we can only control what we are aware of, we are much more likely to control and reduce the use of those things that we measure. Devices like tachometers are useful because they give us real-time infomation about the amount of energy we are using. Similarly, home energy meters, which allow us to measure the electricity usage of a specific appliance or tool, are becoming a popular way to make people aware of both the cost and potential impact of the household energy that they use. Putting a dollar cost on the running of the old refrierator, freezer, or furnace creates a powerful incentive to change to something more efficient. Measuring our total energy use amounts to a rather shocking wake-up call about the rate at which we are burning up limited fossil fuels and overheating the atmosphere. New techologies are great and will help us to move away from the fossil fuel economy. But there's no need to wait. We can save a lot of fossil fuels right now just by becoming more aware of our energy use, and then changing our behavior to save ourselves money, while at the same time we help to save the Earth.