Join the Car Talk Community!  Can you please explain why my
tachometer reading would change...  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 t