the fourth gauge

[Fly By Night]

monitor the speed of the driveshaft

Exactly, otherwise you have to do math based on your speedo reading, then factoring in tire size and gear set.

OR Biased off ones tach then multiply through the ratio of the gear you are in.

Its much easier to just look at the gauge.

 

[powerstrokenstang]

I say oil pressure or water temp.

 

[Dieselboy.]

Fuel Pressure
Pyro for the other bank
Scan gauge - which can do fuel pressure with an add on sensor. As well as EOT and ECT
Back pressure

 

[Dmstrucks02]

I went with fuel pressure, then later added a scan gauge over the steering wheel