MY tach is reading 3400 rpms and gps says 42 mph , their are tach setting on the back of the tach like 864 and pulse settings which settings should i be using its a mercruisere tach any help would be appericated thanks james

Just my opinion, but I would say you should be using the #8 setting as this refers to 8 cyl.

3400 rpm and 42mph sounds about right. And yes you want it on 8. The 8 6 4 refers to the cylinders.

That's pretty close to what I'm running, 3600 and 41-44 on GPS depending. 97 454/330hp