How to Adjust Factory Tachometer?

[badpeanut]

I picked up a factory tach from a big truck and just finished installing it and a reproduction vacuum gauge in my Blazer. Unfortunately the tach feels like it reads a few hundred RPM high. I have HEI and a late 1970's Camaro tach filter. I'm going to temporarily hang an aftermarket tach under the dash for comparison. I read in other posts about the calibration screw on the backside of the factory tachs:

1) Can I reach this screw without removing the gauge cluster (is the circuit board in the way, do I need Ethiopian arms, etc)?

2) Is this screw a flat head or Philips head?

The tach currently sits nicely at zero RPM with the ignition off.

[badpeanut]

I hooked up this old (50+ years old according to Snap-on tech help)tach/dwell meter first to my other truck with a new Sun tach on the column. Difference was a consistent 200rpm across the board.

Next I hooked it up to my Blazer with the factory tach; the factory tach is 500-700rpm low and the difference gets even greater with rpm.

[brad_man_72]

If you have a 6 cylinder engine and a v8 tach it will read 25% lower at all rpms.
If you have a v8 with a 6 cylinder tach it will read 33% higher at all rpms.
I may have my % off, its been a while since math class.
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