That means you need to get toa gas station.
If it is your voltmeter that drops while the vehicle is running. Most likely the alternator isn't putting out enough volts.
speed sensor
It should maintain aprox 14 Volts, give or take 1/2 volt while engine is running.
Check your electric fan operation and coolant level. If the fan is not working, temp will increase at idle and will decrease when air moves across the radiator while moving.
It's probably the Voltage regulator inside the Alternator.
To prevent the stomach drop feeling while driving, you can try sitting in the front seat, focusing on the horizon, avoiding heavy meals before driving, and taking breaks to rest your eyes and stretch.
Well, kinda need to know if it did anything else. If the vehicle continued operating at normal, probably a fault voltage gauge or regulator. If you actually lost all power, the whole thing would shut down. If it was a temporary loss of power, then it kicked back to normal, you're probably looking at a loose or faulty ground wire.
For drop C tuning, a common and recommended string gauge is 11-56.
For tuning to drop C on a guitar, a string gauge of 11-56 is recommended.
While it is possible with a spindle drop....Why would you want to drop a 4x4 truck?
The recommended string gauge for drop C tuning on a guitar is typically 11-54.