October 5, 2005

Measurement vs. Rote

For reasons best left unstated, I was driving home the other day in a Ford Taurus that recently belonged to me. While doing so, the reading on the tachometer forced its way into my attention, likely because the instrument was placed differently than I am used to on my regular car and because the value it was reporting at idle differed as well. I noticed something odd about said tach, which bothered me so much I'm still thinking about it, and here we are.

The tach is extremely standard - it's an analog dial, with white markings scribed along the outside rim. Its major (labeled) tick marks indicate RPMs x 1000. This makes sense. What didn't make sense to me was that the unlabeled tick marks were placed such that there were five between each thousand point. I was at a loss to discover why this was so. The purpose of the tach seemed to me to be to measure the RPM of the engine in a manner that allowed it to be compared to numbers in a reference as well as compared to other values from experience. Sure, it's not going to be the most accurate thing in the world, and people are going to estimate and round. But if I have performed my crappy arithmetic properly, each of those tick marks denotes 166.6-bar RPMs.

What good does that do me?

Sure, the *middle* one is 500 RPMs. Woo. But why *two* on either side?

I know, I worry about stupid stuff.

"What's your engine idling at?"

"Uh, eight hundred thirty-three bar."

Which it was, too.

Posted by jbz at October 5, 2005 11:20 AM | TrackBack


It is no more stupid than the whole idea of putting a Tach on an automatic transmission in the first place. What the hell do you need it for if you don't have any control over shifting anyway? I still don't get that, but Detroit keeps doing it.

Posted by: Tobias at October 10, 2005 11:46 AM
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