I read your recent column about the relative uselessness of...

Dear Car Talk

Dear Car Talk | Dec 01, 1994

Dear Tom and Ray:

I read your recent column about the relative uselessness of tachometers, especially with automatic transmission. Well, I'm a deaf driver and I showed the column to my hearing impaired husband. The tachometer lets me know the engine is running after I start the car, thus preventing me from grinding the starter. My husband and I drove into the mountains this past weekend, and I have a question about something the tachometer did. We were on a steep incline headed for the mountain top. I had the automatic transmission of our '87 Ford Escort in Drive. The car strained to get up the curving mountain road. Most of the time, the tachometer needle was on two, but at intervals, it jerked up to three or three and a half before returning to two. At the times the tachometer needle jerked, the car also jerked as though the engine was getting a second wind. Since we are deaf, my husband and I can't tell you what the engine sounded like, or if there was any backfiring, but the car has been running fine ever since. Can you tell us what was going on and how serious it is?

TOM: It's not serious at all, Mary. If your car DIDN'T do this, it would be serious. What was happening is absolutely normal; the transmission was "downshifting."

RAY: If you've ridden a ten speed bicycle, you know that if you're riding along in say, tenth gear, and you suddenly come to a hill, it gets hard to pedal. So what do you do? You shift to a lower gear. And when you downshift to say, first gear, it gets a lot easier to pedal, but suddently your legs are moving at warp speed! Well, the same sort of thing happens in your car.

TOM: If you're going up a hill in second gear, the engine speed---as measured by the tachometer in revolutions per minute---may be 2,000 (your tachometer abbreviates it as 2). When your automatic transmission senses that it's getting too hard for the engine to "push" in that gear, it shifts to a lower gear. And suddenly, in first gear, the engine is turning at 3,000 rpm, even though the car is going the same speed.

RAY: So that's all it is. When your transmission shifts to a lower gear, the engine speed, as measured by the tachometer, increases. And when your transmission shifts to a higher gear, the engine speed decreases.

TOM: And by the way, you two may be the luckiest '87 Escort owners on earth. You get to drive your car without having to hear the hundreds of OTHER noises it makes! 1903

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