Cable-driven Tachometer Accuracy
Thread Starter
Instructor
Joined: Mar 2017
Posts: 131
Likes: 3
From: Beaverton OR
Team Owner
Joined: Jul 1999
Posts: 65,492
Likes: 230
From: Orange Park Florida
As for overall accuracy, I took a digital audio oscillator and a Techtronics scope to the car once, trying to find out about the accuracy.....
and I found the electronic tach read some 600 rpm high around 4k rpm or so.....
and part of it was getting fried on that silly tire speed/rpm thing on some tire company web sites.....they are RONG!!!!
the speedometer tends to be optimistic in the readings they honest on my '72 until 80 mph which is really only 78 via GPS.....at 100 indicated mph, it was really 94 mph.....ran outta room, so test over....
methinks the tachs are set that way deliberately, same with speedos.....
Pro
Joined: Nov 2015
Posts: 600
Likes: 110
From: Mainz
You can have them rebuilt. The spring gets weak over time as well as the magnet. Mine was pretty accurate after rebuilding as long as I was normally driving. But when racing it lagged a good bit. Once on a chassis dyno we compared readings and it was about 600 RPM low at high speed. (6800 vs 7400)
JIM
JIM
I know this thread is a few months old, but here is my experience with my 71' cable driven tach...
At idle and low rpm it is pretty acurate (at idle about 80rpm of), but at high rpm it gets really wrong readings... My rev limiter of my pertronix 3 is set at 6300rpm and should be +/- 50 rpm acurate. When I was acelerating quick in the low gears and the rev limiter kicked in at 6250/6300 my cable driven tach showed 5700rpm! (not that I constantly rev my engine that high, but I wanted to test it)
Last edited by Alex66; Sep 8, 2017 at 03:29 PM.
Safety Car
Joined: Oct 2004
Posts: 3,920
Likes: 731
From: Youngstown Ohio
I know this thread is a few months old, but here is my experience with my 71' cable driven tach...
At idle and low rpm it is pretty acurate (at idle about 80rpm of), but at high rpm it gets really wrong readings... My rev limiter of my pertronix 3 is set at 6300rpm and should be +/- 50 rpm acurate. When I was acelerating quick in the low gears and the rev limiter kicked in at 6250/6300 my cable driven tach showed 5700rpm! (not that I constantly rev my engine that high, but I wanted to test it)
At idle and low rpm it is pretty acurate (at idle about 80rpm of), but at high rpm it gets really wrong readings... My rev limiter of my pertronix 3 is set at 6300rpm and should be +/- 50 rpm acurate. When I was acelerating quick in the low gears and the rev limiter kicked in at 6250/6300 my cable driven tach showed 5700rpm! (not that I constantly rev my engine that high, but I wanted to test it)
