Well I have the boost/vac, a/f gauge, water, and oil gauges. Dual pilar pod with a 2 pod on the dash. People always say that the a/f gauge is really useless and it just jumps around and doesn't read it right, but honestly from my experience it has helped me out a bit. AT my tune, I saw it was running too lean on the gauge, so they fixed it. Then I found the vac leak i had, and saw the car went really rich on the gauge...which was correct as my dyno proved it...so I actually take good use in that one.
The FP guage isn't very helpful IMO unless you are going to heavily modify you car and need to monitor that. And of course that Nitrous gauge you won't use, unless you put nitrous on. The base ones that I would recommend to get are the water...this guage pretty much saved my engine (new engine). I was driving and no smoke, not nothing on the car, glanced at the gauge and it was starting to overheat, i drove it very carefully and got it home when it was about to overheat! SO WATER TEMP I SAY GO FOR IT
Then I have the oil one...very helpful, monitors the pressure of oil in your engine and we all know that oil is the BLOOD of a car....when that goes, bye bye engine! (same with water!) So those 2 are a must IMO.
As for the rest, its a matter of preference.
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