I noticed from a story in the paper the other day that car sales in America are down. The cause of that phenomenon is no mystery to me. I blame tachometers.

Many people believe revolutions per minute is a measure of political instability in Third World countries, as in, Mali has had 4,000 revolutions per minute over the past two years. Actually, this is not the case. Revolutions per minute have to do with your automobile.

You'll be surprised to learn there is a gauge on your dashboard that reflects how many revolutions per minute are going on under your hood.

That is called a tachometer, which most people thought had something to do with how fast Mexican food was being consumed. For example, some car owners have hypothesized that, when the needle on their tachometer was waving back and forth like a windshield wiper in a monsoon, it meant your engine was burning up gasoline at the equivalent rate of 6,000 tacos being eaten. When you stepped on your gas pedal by mistake when your car was in neutral, a lot of tacos were going down the hatch.

The truth is that a tachometer tells you how many revolutions per minute your engine is turning. If you look closely, as I did, you may be confused at exactly where all these revolutions are taking place. I had my wife rev up the engine one day while I stood there with the hood up, and I didn't see my engine revolve once. From that, I surmised that it isn't the engine that's revolving at all, but something much more subtle is going on in there. What that is, I have yet to observe.

Drivers today have little interest in revolutions per minute, either in Mali or in their cars. Yet, as Frank Sklenarik, a longtime observer of many interesting things around here correctly pointed out to me recently, Detroit continues to charge you a lot of money to keep you posted on that arcane statistic.

For some reason, automakers think they're doing you this inestimable favor by enabling you to know how many revolutions per minute are going on in your engine. It may be too late to break the news to them, but nobody cares, and very few even know what a revolution per minute is.

There was a day, I guess, when this field of measurement had some significance. Its heyday, as I understand it, was in the 1950s and '60s.

This was back when gasoline cost 30 cents a gallon and the idea of driving, especially by young men, was to arrive at the speed limit as quickly as possible and stay there as long as possible. (Some drivers might even have been provoked into exceeding that barrier.)

Cars then came mostly with manual transmissions, which required the driver to shift from one gear to another. It was very quaint.

To make drivers think they were Richard Petty, the automakers would include this gauge to measure revolutions per minute (which they conveniently shortened to "RPMs") as an incentive to step harder on the pedal between shifts. The strong suspicion was that gasoline companies were in cahoots with the car companies because they were moaning that the price of gas was too low, and they had to figure out a way for customers to use more of it.

So they put out the word that, if you shifted your car when you had any fewer than 6,000 RPMs, you were wrecking your transmission and it would fall out on the ground any minute.

Well, we all fell for that one, and pretty soon everybody was paying attention to RPMs.

However, drivers soon tired of all that exercise, and gas companies were beginning to get their way on prices, so who needed all that shifting? The automatic transmission gained a solid foothold, and before long RPMs were relegated to record turntables.

Unbelievably, though, Detroit wasn't paying any attention and kept putting tachometers in their cars, even though nobody was doing any more shifting and completely lost interest in RPMs. Installing tachometers became such a habit that it continues today, unabated.