Years ago the speedo stopped working and I was using a Garmin GPS for speed readings so I didn't need a speedometer.
I installed this manifold gauge from a WW2 P-40 Kittyhawk and while you wouldn't think a manifold pressure gauge is that useful in a normally aspirated engine they can be useful in detecting engine problems and I thought it looked cool and different. I had literally hundreds of these old WW2 and post war gauges laying around at work a few years ago after buying a package of military surplus (now sold) and they can be picked up on eBay for $20 to $50
It was useful for detecting manifold leaks and I could even tell when the dampening oil needed topping up in the SU carburetor I was using on the Hurricane I4
I also had installed in addition to the usual water temp, oil pressure, amp gauges
1. Fuel pressure , a very useful addition for diagnosing no start or running problems
2. Gearbox temperature, you'd be surprised how hot the transmission and transfer case gets in heavy mud
3. Engine oil temp , sensor mounted in the sump plug. Oil temp in my experience is pretty close to water temp +/- 5 deg C , a spiking oil temp can sometimes give you an early warning before a serious engine damage