hey guys when i am drivin at nigt and i have my lights on and i put the radio on with the subs my speedometer goes nutty, like if i hit the gas it goes to 0 then when i let off it goes to the rigth speed. now my guage says 13-14 still so i dunno wuts goin on here. if u have any insight thanks

 

I had a similar problem, when the lights were on the tacho went down, and when I indicated the tacho went up and down in time with the indicator. I pulled the instrument panel and one of the copper wires on the back had broken. This meant the lights and indicaotor warning bulbs were earthing back through the tacho, instead of to ground. I fixed it by solderng a bit of wire across the break.

I have a mechanical speedo, so it was OK. But is sounds like you may have somthing similar that causes the voltage across the speedo to drop when you have the lights etc on.

I always check the earthing first, as that is the most likely place for electrical probs on the older cars and cheapest to fix. After that you have to check the volts to the panel and see if there is resistance in the system, causing the drop. You don't need much of a drop to mess up the instruments.