IMHO, the least useful gauges in order of uselessness:
1. Air Temp Gauge You are sitting on the outside of the bike, not inside a car where it might make sense.
2. Voltage Gauge Not terribly accurate in most cases, and most riders don't really know how to use it anyway.
3. Oil Pressure Gauge When the pressure drops to less than 3 psi, the red idiot light comes on. The gauge causes confusion for many riders, more often than not, and a lot of unnecessary angst.
4. Tachometer Many of us old geezers got along for decades without a tach, since most bikes didn't have one back in the day. Now that Harley's have an effective rev limiter that will keep you from overcooking the engine, it's pretty much superfluous, just like they are in most cars.
5. Fuel Gauge A properly calibrated gauge beats the heck out of the old trip meter method. Unfortunately many Harley fuel gauges are not properly calibrated, but they can be fixed. As for the miles to empty thing, often they are not any more accurate than the gauge, since both are based on the same signal from the same sending unit.
6. Speedometer Who cares how fast they're going? Not that many based on my observations out on the highways around here, where anything less than 80 will get you passed constantly, even though the speed limit is 55.
Just my opinion of course, but of the four small gauges on a Harley Touring model, the only ones I would keep are the fuel gauge and the oil pressure gauge, and that's just to keep from having to fill all four holes with something else to cover up the holes. You can always add a couple tweeters to two of the holes, not sure what I'd use to fill the other two..
Jerry