When was tach and cable ever checked, how long has cable been installed tach hours and calendar time? They can become worn and bind due to lubrication. Try using a digital handheld tach and compare readings. Don't be surprised if mechanical tach reads maybe 50 to 100 low as they can bind.

Bill

install a digital tach
some are primary replacement
all are very precise
they are coupled to both magneto and thus can give you indications when something is not perfect on mags,this in addi8tion to giving you a lighted andprecise reading.
mecanicals tach are things of the past

Good stuff Daniel I'm on it!

Mechanical tachs are nothing more than cheap speedometers with a spiral wound cable that turns the instrument pickups thru a flexible housing.  They sometimes "wobble".  They sometimes can be fixed with pulling the cable and lubricating it with graphitic grease or replacement.
In any case, keep this in mind.  The instrumentation is not critically accurate, nor is it necessary that it be.  It's just a rudimentary way to generally tell what kind of power setting you're pulling.
If it really bothers you, you can throw a little money at it, ...or a lot of money at it.  It won't make your engine run any better.
(While I don't like "wobbly" indicators, I fix them, but I don't usually just jump into it with a full wallet.  I usually just replace the broken/weak part with similar parts.  My own experience with "digital" instruments is that they are a tedious detail.  Who cares if your EGT is 1437 degrees and "wobbling" between that at 1439 and 1428 and on and on and on ad distractum.  And who cares if your mag drops 47 on a digital ...or about 50 on an analog that costs 1/4th as much.)
But that's just my nutty, inexpensive, opinion.

if you can Dave ,get a digital tach.unless you are from a realy old school you will appreciate it .                         .i love all my digital instruments
we have found out,that  in the demanding skydiving operations ,that having precise readings in tach, egt ,cht,we can find the best operating procedures and extend cylinder life.
i dont know in states but here in canada we have to have a yearly check on rpm precision if its out by 100 you have to repair .
in mecanical  tachs the more you wear the more imprecise you get .

euh ! 400 rpm?
no sorry thats my harley.                                                            i have no idea.
do i have a pilot licence ?                                                           let me check that too

Answer: 2200 RPM -2450 is the green ard,  2450-2700 is the yellow arc, and 2700 is the redline.  Digital tachs don't display that info.

George,  you may have just come up with an idea that will make your fortune. 
Perhaps you should approach the digital manufactures with lights for the various rpm ranges. 
If I had a digital tach, it would be nice to have colored LED
lights to represent the ranges, in addition to a digital rpm readout.

Hmmmmmmm-  Sounds like I may want to investigate these digital tachs a little more.   I have not had the oppertunity to see one in action.
Thanks for the info Daniel.

youre right dave
when you order at horizon they requesy for wich very specific airplane and unless you tell them differently they are programing the gauge for you .
the green ,
+yello(as you say )+RED LIGHTS WILL BE ON DURING THE RANGE SPECIFIED BY THE MANUFACTURER .
in addition they will write on the face of the instrument the rpm range for each of the categories (.ex.green 2200.2500)

the thing also act as a timer for different purposes,including time of cureent flight etc etc .

I put the Digital in and enjoy the benefits of both -

I find it easier to maintain Altitude because the digital picks up on minor changes.

I like the indicator lights for the mags - if you have an open ground on a mag - weather it be on the runup (intentional)  or some other time (unintentional)  You get an immediate red light.

I also like the analog or dial and have retained it - I've gotten used to seeing the dial in certain areas, and on initial power changes I like to see the dial position.

The Digital has the lights for rpm ranges so they'll remind You of operating ranges green-yellow----RED- just in case you weren't paying close enough attention to the dial at that moment.

They both have their merits - so I'm retaining both.
- and could recommend the same to anyone else

Ken Wanagas