Factory tach mystery

[Eastonspeedworks]

So I have a rather odd issue with a factory tach/air pressure gauge I recently purchased. I previously had a sunpro tach I added into a stock tin that worked very well but didn't look quite factory. So tonight I installed the new dash everything went smoothly I got it all hooked up and everything was testing good until I started it to check the tach. It said it was running 500 rpm idle. I knew this wasn't right and the problem became more apparent as I revved the engine. The tachometer wasn't climbing like I was expecting so I hooked up my hp tuners software as its an ls engine and I confirmed my fear it was reading unproportional. Initially I thought I was sold a 6 cylinder tachometer not an 8 cylinder one. But looking at the numbers it's backwards the tachometer says 500 and the computer says 760 ish. The tachometer says 4500 and the computer says 6000. I did some research and found they can be calibrated with a screw on the backside but that seems to only change a set rpm. Anyone have any ideas? I'm a bit bummed out I spent an awful lot on this dash to go back to my old one