Is there a adjustment on the older gaffrig tachs to calibrate them? Mine are off. At idle and about up to 3000RPM they are even. When I get going say around 3800RPM, and synch the engines by ear, they are about 3-400RPM off from each other. AT wot one reads 5100, the other 5500. Im guessing the one reading 5500 is off otherwise my slip numbers would be over 20%. I plan to hook up a buddies fluke meter with the tach function to see whats goin on. Just wanna see if theres a adjustment or if i need to replace a tach...

or you could just switch the leads and see if the problem switches as well

I thought about doing that. But figured it may just be easier to hook the fluke meter to it. Obviously one tach is off, but which one is the question. Its a new to me boat, and one of the tachs looks as if it was replaced more recently, as the pointer needle is bright orange. Other one is totally faded out.

I don't know about gaffrigs, but every tach I've worked on had an adjustment - only way to get them reading correctly at the factory. Look for a plug or small decal on the back to see if there is an access to the adjustment. Unfortunately, some of them are sealed inside the case with no access hole, so the only way to adjust is to remove from the case (which may tear up the tach).

Unless you have an ossilliscope, a signal gennerator or a known good tach it will be a guessing game...easier to just send it back to Gaffrig and have them callibrate it.

The Fluke meter is pretty accurate - I'd hook that up and adjust them both to read what the Fluke says if it was mine. Onboard tachs are indicators anyway, not accurate instruments.

I had the same thing going on I replaced both ,I had gaffrig /livorsi and gaffrig couldnt match them and livorsi needed the gauge for two weeks to repare I found two new ones on ebay that matched its nice to have tachs again. livorsi said they could fix it I just didnt have the time to wait