Tach Verification

[Phil Holmen]

Is there anyway to verify that my tach is reading correctly??

[turbovair]

I knew this Auto Math book would pay off some day! The mathematical formula is:

RPM= MPH X GEAR RATIO X 336 divided by tire diameter
For example:

RPM= 60 X trans final drive ratio X rear axle ratio x 336
------------------------------------------
tire diameter

some hypotheticals:
60 X .68 X 4.10 X 336
---------------------
28

Tachometer should read about 2007 RPM
Hope this helps!

[DOG]

tach runs off alt.computer uses crankshaft sensor.hook up a scan tool to see rpm in the data and compare to the tach. I just did this recently tach was out by 500-600rpm ordered proper pulley from dealer there is only 1 listed for 6.5td.Dia was larger than the aftermarket(rebuild)that is on my truck so tach reads better now BUT now its not charging properly because of slower alt speed.Only sollution I can see is to replace alt with proper AC delco.pulley $25 AC alt@$200cnd