Can you please explain why my tachometer reading would change...

Dear Car Talk

Dear Car Talk | Dec 01, 1994

Dear Tom and Ray:

Can you please explain why my tachometer reading would change for a constant speed driven? When I first bought my car six years ago ('88 Mazda MX-6), the tach would read 3,000 rpm in fifth gear at exactly 70 mph. It was so reliable that I could tell how fast I was going by simply looking at the tachometer. Now, at 70 mph, it reads 3,100 rpm. Should I be concerned?

RAY: No. There are kids starving in Somalia. There's a health care crisis in the United States. My brother has a huge rash on his rear end. These are all things to be concerned about, Ken. But I wouldn't worry about your tachometer.

TOM: If your rpm were varying a lot at highway speed--if they were going from 3,000 to 3,500 without the car going any faster--then I'd be concerned that your clutch was slipping. But the clutch would never slip to only 3,100 rpm. It would never slip that accurately and that predicably.

RAY: My guess is that your tachometer calibration is slightly off. And that can happen over the years. It may be the sensor, or the instrument itself. But it's really nothing to worry about.

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