voltage drop

Those numbers you got the 13.2 and 9-10. This is with the car running? Sounds like you have a draw on it.

But were are your grounds to. On my car I have a ground from the battery to the frame. Then I ran another ground from the spot on the frame to all the way up front were the stock ground was, then ran another ground to the engine. I use welding cable for both positive and grounds. I had got some from my job and just strated using it up.

Then on the positive I ran from the battery to the cut off switch. from the otherside of the cut off switch to the coil then from the coil to the starter. Also using welding cable for everything. It's probally over kill but I just had so much cable lol.

You need to use a DMM for this, not only for accuracy but to not be introducing a lame stock gauge, it's wiring and connections as possible variables.

As noted, you'll want a ground from the neg terminal to the stock grounding location up front.