New tach, needs calibration - General - VWDiesel.net The IDI, TDI, and mTDI source.

• New tach, needs calibration by erice1984 on 12 Feb, 2011 07:15
• So I have my tach hooked up to the W terminal on alternator.  Now I want to calibrate it so it reads true RPM to the motor.
  
  In the bentley manual is says using an adapter and dwell/tach meter.  Any idea where to obtain the adapter and what dwell meters would work with it?
  
  Thanks
  -Eric
• #1 by Vincent Waldon on 12 Feb, 2011 09:24
• Using a dwell meter with a vibration adapter is one way to do it... but one that was developed 30 years ago.
  
  Nowadays a cheap laser tach from Harbour Freight / Princess Auto is probably a cheaper and much more accurate approach.
• #2 by erice1984 on 12 Feb, 2011 09:49
• the insight i was looking for, thanks Vince.
• #3 by bajacalal on 17 Feb, 2011 20:42
• There is a way to figure it out using arithmetic, basically you can calculate your engine RPM if you know your speed, gear ratios and tire diameter, assuming it's a manual transmission...  You would figure out how much you are off, as in what %, and adjust accordingly.
  
  I can't help you with the math but if you google something like "engine RPM calculator" and look here you will probably find what you need:
  http://www.brokevw.com/020ratios.html
• #4 by erice1984 on 17 Feb, 2011 20:44
• I am sure you could, but I don't have time for it right now. lol
  
  I kind of guessed at what it was.
• #5 by Vincent Waldon on 17 Feb, 2011 22:03
• In case Search ever leads someone here some day, this chart shows the gears ratios of common 020 trannys and then lets you plug in tire size, gear ratios etc and figure out RPM at MPH or visa versa:
  
  http://www.scirocco.org/gears/