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Why you should never buy a car without a tachometer
Over the years I've owned my current ride, I have observed that it is most
fuel-efficient at 3500RPM, where 42MPG is easily attainable. I have also observed that, at 3000 RPM or less, the mileage on my vehicle drops to 35MPG or lower. My experience with my current vehicle is similar to several other vehicles I've owned. In every case, I've found that the best fuel economy is achieved somewhere above 70MPH. In the case of my current vehicle, 3500RPM in top gear equates to about 78MPH. Now I'm sure someone will state the obvious, which is how do I know that my tachometer and speedometer are accurate? I don't. But the bottom line is, in top gear, I can achieve BETTER fuel economy at slightly higher than normal highway speeds. Actually, the better fuel economy is due to slightly higher engine RPM, and the higher ground speed is COINCIDENTAL. I've always known that every engine has a "sweet spot", but didn't know the exact scientific explanation for my personal observations until just yesterday. During research on various engine types, I happened to discover that Otto Cycle engines are most fuel-efficient at RPMs near (not necessarily at, but near) 40% of redline. I had no reason to disbelieve that particular scientific fact. I did the math for my own car, and found that (surprise, surprise) I SHOULD achieve maximum fuel economy at ABOUT 3200RPM, with my own Otto Cycle engine. This agrees almost exactly with my own observations that 3500RPM (43% of redline) is very fuel efficient, and 3000RPM or less (37% of redline or lower) is not as fuel efficient. Obviously, my engine is very fuel-efficient near 40% of redline, AS IT SHOULD BE, and it is less fuel-efficient at lower RPM levels. Note that I had no idea that my engine SHOULD be most efficient at about 40% of redline before I observed that it WAS most efficient at 3500RPM. So the results I observed were not results that I "expected" to observe. The facts are what the facts are, and now I have the scientific explanation stating YES, it should be that way. From what I could gather, the reason for the lower fuel efficiency below ~40% of redline in an Otto Cycle engine has to do with valve timing and other aspects of engine performance. Most Otto Cycle engines have fixed valve timing, so the maximum fuel economy is achieved at a certain RPM, and if you run the engine higher OR LOWER than that RPM, your engine is not achieving maximum fuel efficiency. Some engine manufacturers have come out with variable valve timing Otto Cycle engine designs, with limited success in improving fuel efficiency at lower RPMs. I gather that the success is "limited" because fixed valve timing isn't the only factor making an Otto Cycle engine most efficient at a certain RPM. Anyway, now that I know WHY I'm seeing fuel economy peak at certain RPMs, I know exactly what to look for in my NEXT vehicle. IF it has an Otto Cycle engine, it WILL have a tachometer, or I won't buy it. During the test drive, I'm taking it out on the highway to see how fast the car is travelling at 40% of the engine redline. The way I USUALLY drive, a car would be most efficient for me if it hit 40% of the engine's redline between 70 and 75MPH. Ironically, my current car is designed for a less conservative driver than its current owner is. While I do get close to 80 sometimes, the vast majority of my highway driving is in the range of 70-75MPH, so that is where I need a car to be most efficient. SIMPLE, right? So if you want to find a car that will be very fuel-efficient for the way you drive, simply look at the tachometer. Wherever it ends, muliply the highest number by (.4). This will give you a good estimate of the most fuel-efficient speed (RPM) of the engine. Then take it out on the highway and see if the engine will run at that speed (RPM) at the speed you normally drive on the highway. If the engine RPM is too high OR too LOW at your normal highway speed, you might want to find a car with an engine that is better designed for the way YOU drive. That is, if your goal is to find a fuel-efficient vehicle. -Dave On a side note, it's easy to understand why manual tranny vehicles are more fuel-efficient for use in City driving. You have to get the engine RPMs UP to maximize fuel economy, and you just can't do that at low speeds in a slush-box. |
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