This spring I think it'll be time to put my extra DIN slot (98 legacy) to work. I'm thinking a set of three gauges will do the trick. One will certainly be transmission oil temp, to keep an eye on my rebuilt transmission. I'm considering engine oil pressure and outside temperature for the other two, but I'm open to suggestion. The gauges I was eyeing were the VDO Vision ones on egauges.com What would be the most useful knowing what the car is up to?

I would vote for a Vacuum Gauge.

 

I don't have one on my Subaru, but have installed them on other vehicles. The vacuum reading is useful, amongst other things, to help conserve fuel. A slight change in throttle position can change the inlet vacuum 'pressure' considerably. And this affects fuel consumption dramatically. Especially useful on long highway trips, but I find it helpful all the time, even around town.

 

Just tee into any vacuum hose somewhere in the engine compartment, preferably near to the inlet manifold.

In the SHO I have:

Volts (no stock gauge)

Oil Pressure (no stock gauge)

Water Temp (no quantitative stock gauge)

 

Others I considered:

Exhaust Temp

Air/Fuel (good for turbo cars)

Thanks folks. I think oil pressure is a winner, since if I know it's dropping quick there's big trouble. I think the other one will be voltage or outside temp - it's nice to know when it's below the freezing point outside in winter.