On our '00 ('99 chassis), I use RPM, Engine temp, average & instant MPG. Or I might select IA (Incoming Air temp) to monitor the outside air temp.
We have no tach in the instrument cluster (Bad Ford!) but we've found the other gauge one might think they depend on - the temp gauge. It is an idiot gauge - it moves from the bottom to a specific point as the engine warms up. When climbing hills on hotter days, the clutch fan kicks in so we know it is running warm. The SG shows the normal temp of about 195° (if memory serves) but will range up to 215° or so before the clutch fan kicks on.
The temp gauge in the dash never moves beyond that specific point. (Double bad Ford!)
There are other devices for the smart phones, but I like a dedicated device. It is compact and fits in where I've placed it:
