Gauges are good, they tell you good stuff.
Oil pressure is a great gauge, saved my ass once too. (long story, for another time)
Trans temp was nice, but basically un-needed.
Aeroforce is the gauge to get....hands down, most awesome thing I've ever had in the GTP.
A wideband is also another one of those, you really outta have, since it displays A/F ratio, and will tell you lots on how lean/rich the engine is.
Water temp...doesnt the car have a water temp sensor already? Thought that was like, standard on every car nowadays...Unless you're going for an exact temp degree.
Boost gauges are fun, you can make the needle go ape**** too! Plus, it'll let you know about any vacuum leaks, and boost issues.
Installs are pretty simple. Each gauge comes with instructions, follow them. The Wideband is probably the hardest install. Everything else is:
Plug the sensor in.
Run a wire from the sensor to the gauge (sending wire)
Run Remote power to the gauge (+12v switched)
Run ground wire.
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Profit...Hell no, you're modding a car, there's no profit to be had!