Use diodes to obtain lowered DC voltage?

I am building a digital voltage gauge for my motorcycle for fun. I am buying a nice little display which has common ground, so that I can provide 5 and 12 volts from the same source. I was hoping to use a "7805" regulator to control the 5v, but do not want to provide the varying 12+- volts directly to the 7805. I would like to drop about 7 volts before I apply the power to the 7805. I looked at Using diodes to limit current to LEDs which seems to indicate that using diodes forward voltage drop is not a good idea for leds, and maybe using a zener diode is not either. If I just use a series resistor, I would have to guestimate and experiment to get in the ballpark (as I don't know the current draw), and, as I understand it, the voltage dropped over the resistor would vary with the applied voltage. Here are my questions:

1. Is using a diode(s) to drop the voltage independent of current a bad idea here? (Is it always a bad idea?)


2. Do I still get he same IIR loss for the voltage dropped whether it is a resistor or diode?



3. Is there a conventional "best practices" approach I should be using?


4. Lets go out into left field for a minute: other than heat-sinking the 7805, can I pot the whole mess with RTV silicon to vibration and moisture resist?

Always nice to know if the alternator is working OK. Not an electronics guy, just a tinkerer.

Thanks!

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Thank you Nick and Thomas. I agree that a motorcycle is about the harshest environment one can choose-greater vibration, direct exposure to rainwater, probably greater variations in regulated voltage due to greater variations in engine (and so alternator) speeds. I had hoped to fuse and put the dropping resistor at the battery connection to provide the greatest protection for anticipated faults in the wiring run and/or the digital readout itself. Based on your comments, I will fuse at the battery connection, run fine (#28?) wire to a MIC2945 and let it do all the voltage dropping. I anticipate a mounting plate fastened directly to the 2945 heat sink, with the entire remainder of the assembly "glued" to the mounting plate with RTV silicon. Waterproofing is a concern, for which I will rely on the RTV silicon.