As long as I can remember, when driving at night in the 914, my center console voltmeter hovers right around 12V when cruising with the lights on. When I hit the brakes, it drops well below 12V...anything else drops it even lower.

I'm running an Optima Red Top near the end of it's service life and want to make sure my charging system is healthy before popping on a new AGM battery.

I had been considering switching to LED headlights and brake lights to lower the current draw.

I hooked up the VOM at the battery and did a load test with the car running, adding an accessory at a time.

With headlights, brake lights and aux fan on the voltmeter in the center console read just over as 10 volts, while the VOM hooked to the battery showed 12.58V. Is this discrepancy normal?

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perfectly normal and as expected. . old wire with poor connections, as well as bad grounding lugs are the most probable cause. . each connection sucks out a few 1/10 of a volt till they add up. . .
Check the main battery ground and positive leads 1st, then work your way vack thru the wiring. . don't forget the trans ground strap

Nothing to do with old wires! They worked like that when they were brand new!

Ohm's law. Voltage drops as current passes through a conductor, increasing with resistance of conductor. Measuring at the battery, very little resistance between source and measuring point, so no voltage drop.

You also could have lost the ground connection within the appearance group wiring. Had the same problem. All is well after I reconnected the ground wire

I added a new ground wire to mine and that fixed it

Yes. This is why I decided to remove the center console and it's gauges to get some more side-leg room. I'll rely on the equally-accurate idiot lights.

I'm fighting the same issue now too, and it was at the point where I thought my alternator was failing because I was only seeing ~10V at the dash even with a brand-new battery. It could be the gauge but way more likely the wiring harness and/or a ground. I was super happy it wasn't the alternator...

The volt meter in the center console is nowhere near accurate. You can use it to see if your brake lights are working (voltage drop when you step on the brakes) and it will twitch with the turn signals, but it being an accurate measurement of voltage, well forget it.

The gauge is connected to the same power source as all the other instruments. So it has lots of connections and other stuff in the circuit to affect its performance. If you want it to be more accurate, run two heavy wires, one red, and one brown all the way back to the battery. Use a relay to switch the brown one off when the car is not running. After doing that, it still may not read correctly, as the calibration is questionable.

Or just ignore the gauge.

I agree check the aux instrument wiring harness ground connection under the console shift base plate, remove clean and plug back in. Volt meter should read 13.6 at idle with no load

As others have noted the difference is expected in the 914 because of small gauge wire causing IxR voltage drops as loads like headlights, turnsignals are active.

I've measured my gauge separately and found the gauge to be accurate to about 0.1V. Unfortunately where it resides in the 914 wire loom causes the gauge to read very low at times.

Although not an accurate measurement of battery voltage, the gauge does give an indication of the voltage going to loads in the 914. For instance the headlights may only be getting 10V instead of 14V which the alternator puts out at elevated RPM.

I like to say that the voltmeter is there "for entertainment purposes". It's not a great indication of the health of the charging system...

--DD