Incandescent (plain old filament bulbs) can run on lower than specified voltage. They simply won't be as bright. If you ran your stock DRL bulb at 12V, it would be brighter than it is when plugged into the DRL socket on the car. LED bulbs (unless they are built with special controller circuitry) need a certain amount of voltage just to turn on. They're pretty much on / off. At 12V they turn on. Below that, they're off. So those bulbs will not work for our DRLs, since you've found the DRL voltage is 5.5V. The only "way around this" would be to run a 12V supply out to the DRL sockets, and use some sort of relay to turn them on. Probably not worth it (to me, maybe you really really want LED DRLs). I would try to find some different bulbs. Most likely, LEDs will not work in this application (without further wiring modifications). You could try some 6V LEDs if you can find some in the correct socket, but still no guarantee it will work.