thats just it, I dont have instructions for these. I bought them wholesale through a friend as overstock items out-of-box. I've got the wiring instructions on the manufacturer website but no dimensions other than the spec sheet saying they fit a standard marine 3-3/8" hole which I know refers to the outer ring dimension since their smaller gauges state "fits standard marine 2-1/16" hole" and I know for those gauges the proper holesaw is actually the smaller 2" holesaw bit...
oh yeah, also, I went to teleflex's website and they say to use a 3" hole saw....on faria's website, they say to cut a 3-3/8" hole. 3/8" is a lot of difference for these things considering the outer lip is so small...
Although I know its hard to measure at the ring base with a plain old tape measure....maybe you can borrow a set of calipers and get a measurement that way to be sure....or cut a 3" hole in a piece of paper /cardboard first and see it it fits.
Gauge size refers to the face size. You have 2" and 3" gauges and they require 2-1/16" and 3-3/8" holes for mounting respectively. If you use a 2" hole saw for the smaller gauges they won't fit.
Yup -- a standard elementary school quality ruler is all you need. Measure the gauge. You can also buy an adjustable "hole cutter" (not a hole saw) for about $8.00. Adjust it for the size of the gauge and cut the hole. Try it in a piece of scrap wood or cardboard if you are unsure of your measurement.
Of course, you could take you 3" gauge to the hardware store and fit it into a hole saw. The back of the gauge should just fit inside the hole saw. The hole saw will cut a slightly larger hole due to the saw thickness.