We're big fans of sequential shift lights or LED tachs. They allow you to monitor your engine speed without having to take your eyes away from the road by lighting up a series of LEDs with engine speed. It's the same sort of thing you see built into $2000 race dashboards and $50,000 Formula 1 steering wheels.
The Ecliptech Shift-P2 and Shift-P2+ takes our previous favorite - the Shift-i - and extends on it. The main LEDs are the primary attraction, lighting up in sequence as you approach the redline. You can select what that sequence looks like from a series of built-in options, or on the Shift-P2+ you can program your own. It'll dim itself at cruise so you're not blinded by high-intensity LEDs when you're not working the car hard. There's also a small display screen that can give you extra information such as battery voltage or programmable alerts. The Shift-P2+ also allows you to add an analog input to trigger alerts, such as an overheating or low oil pressure condition. All the programming can be done via the keys on the tach.
The Shift-P2 can be upgraded to the Shift-P2+ with a software key.
Works with all Miatas and most other vehicles.