tach issues

[sandawgk5]

Well I finally got my factory tach dash installed but the tach reads high. I read somewhere that if the tach is for a 6 cylinder then indicated rpm will be 33% higher than actual rpm which leads me to the question of. Does anyone here now how to convert the circuit board for a 6 cylinder tach to work on a v8. The tach in question is from a 79-81 chevrolet dash.
Thanks.

 

[dyeager535]

I never found any way to convert them.

Have you verified that at all MPH, the engine speed is off by that percent?

My dad and I both have '81 (or so) tach's, and neither one of them is reading correctly, at least according to any calculations I have done. Even taking into account advertised vs. actual tire diameter differences of up to 2", the tach isn't right. (and all the other variables are known)

But neither is off 25%, which is what you'd see with a 6 cylinder tach on an 8 cylinder engine.

 

[sandawgk5]

I know how I just dont know which resistor needs to be changed. It is kind of weird when I start the truck it jumps to 2000 as it warms up it comes down to 1250-1300 which if I subtract 33% of that I get ~830 this is in park If I put it in gear it drops to about 1000 which is 670 which is about where i would expect it to be. With it in neutral I can rev the engine and it has no problem wraping the tach all the way to what would equate to about 6500-7000. All of the errors are approxiametly 33%. If you take the 33% difference from 7000 the actual rpm would be ~4600. The fix itself is really simple the hard part is finding where to put it. I could be wrong I am just a mechanic not a micro-electrician (if that is a real thing) /forums/images/graemlins/dunno.gif well thanks for the input. If I do figure this out I will be sure to post up.