hmm fair enough...what about the air pump do you still have that? If the spark plugs are not the issue it may have to do with the electronic load detection. Im just trying to figure out what is happening here.

 

I still have the air pump.

 

Damn S/C F20 miata, wuts that thing weigh???

 

sounds like a bad ground, it will reset and read zero if the power is cut off and reconected while the car is moving. The extra g-force after the supercharger install could be causing it, also is it a vortec S/C ? the control box is mounted under the dash on the computer side ( not sure in your case since it's in a Miata ) Maybe the wires got moved.

 

Yes, if I turn off the car while I am rolling and turn in back on in gear the speedo goes back to normal. I also have found that certain partial throttle %'s will make it come back on which is weird. I figure its a bad ground, its just odd that it did not show any issues until now.