Tachometer debate.

Dear Car Talk

Dear Car Talk | Dec 01, 2001

Dear Tom and Ray:

Is the tachometer directly related to your forward speed, or does it vary depending on the incline the vehicle is traveling on, given that you are in the same gear the whole time? We've been debating this question for two days now, and if we don't get an answer fast, someone here is going to get hurt. Can you help? -- George

TOM: Sorry it's taken us a few weeks to get back to you, George, but it took us that long to figure out what the heck you were talking about.

RAY: The tachometer measures the rotational speed of the engine. And nothing else. This has nothing whatever to do with the road, how fast you're going or how many mosquitoes are in the air.

TOM: What I think you're trying to ask us is this: At 20 mph in second gear, for example, will the tachometer always read 2,500 rpm, whether the car is on a flat road or going up a hill?

RAY: And the answer is yes. On a given car with a standard transmission (automatics allow free-wheeling), the tachometer reading in a specific gear should always match up with the same speed on the speedometer. If your tachometer reads 3,500 at 60 mph in third gear, it will always read 3,500 at 60 mph in third gear. This is true regardless of whether you're going uphill, downhill or around in circles.

TOM: And now that that's settled, I suggest you debate something of greater significance. Like whether a man's intelligence is inversely proportional to the size of his belt buckle.

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