95 chevy tach problem the reading is off...??

[J_K_auto]

I put a 97 6bt in a 95 Chevy that had a 6.5. I am using the 6.5 alt

I did my conversion a why back and things are going ok with it till today.

Anyhow I found out my tach is off. I am not happy about this of course

It reads ok near the bottom as it comes up it starts to read off

tach / should be

2500 2750
3000 3500

I think my pulley is wrong on the alt.

I forget why I never used the harmonic dampner sensor but I am wondering if I can use it to test my rpm or use it instead of the alt.

I did search this on here and did see about the dokota digital but trying to stay with what I have already..

Anyone know?

thanks Joe