7805 regulator heats up/fails

Again, that is overkill for this job.

The fairchild part has 55dB of ripple rejection already, and 0.1 % load regulation.

Remember this is a simple hot wire gauge, running on battery power. You could add a capacitor to the regulator output if the 55dB is not good enough.

That fairchild part is less than 2 bucks. And is all that is needed to replace the OEM regulator. No need to spend 80 bucks on a DC-DC convertor.

B.