Miata add-on Fuel Pressure Gauge

I am wondering why it is that a fuel pressure regulator is useless without the "under hood" gauge. Wouldn't a air/fuel mix cockpit gauge be sufficient? BTW, I have a 96 Spec Miata.

The reason we say that is because you should know what your adjustments are doing to be able to tune properly and safely. It's not as "useless" without the fuel pressure gauge as it would be without the air/fuel ratio gauge, because the main tuning measurement is the air/fuel ratio as you mentioned, but we stress the importance of knowing what's going on with your adjustments because it is downright dangerous to tune without knowledge of knowing what your fuel pressure adjustments are doing to your air/fuel ratio. If you lean the engine out too much, it can very easily detonate and destroy your engine (ie: burning a hole in the top of a piston). The fuel pressure gauge under the hood (or anywhere for that matter, we have people that install a dash gauge for monitoring fuel pressure) will allow you to see how much you are adjusting the fuel pressure, and how those adjustments effect your air/fuel ratio. It is a entire system that should be used together. Sure, each component will work individually, as the regulator will work without the gauges, but what's the point of tuning it if you don't know where your adjustments are taking your air/fuel ratio? Also, the biggest benefit of our adjustable regulator is the ability to tune at the track for different weather or track conditions. You'll first need to discover a baseline setting of what fuel pressure yielded the best horsepower numbers and at what air/fuel ratio that occurred at (preferably on a dyno), then record the fuel pressure so you can easily adjust back to the baseline when you change it later (add more fuel for a torque track, or less fuel for a HP track). This is why the fuel pressure gauge is important, as you're tuning blindly if you don't have some sort of reference point. All this is why we say "useless", as we don't want anyone tuning blindly as major engine damage might occur. So to sum it up, the fuel pressure gauge is your "reference point indicator" that allows you to make the proper adjustments without endangering your engine with over-lean conditions.