Speedo reads 20 sitting still and fuel gauge moves toward e when I turn lights on.

[dhughins]

I recently got truck and the electrical part is all hacked up. The speedo reads 20 and goes up when I drive. I'm not sure if it's just 20 off all the time or worse. When I turn on the lights the fuel gauge starts going to e until it's all the way left. Turn off lights and it goes back to where it was. Any of y'all seen this before. I wanna be able to drive truck so I'm not able to completely rewire it. None of the air or heat stuff has electricity to it.

[rich weyand]

You should tell us the year of your truck, as the wiring changed over time.

That being said, the first place to look is grounds. There are four electrically isolated parts on these trucks: the battery, the engine, the cab, and the frame+bed. They are electrically isolated due to the rubber mounts on the engine and on the cab.

So, to connect four things together so they all agree on ground, you need three wires. One is obvious: the big black cable from the battery ground to the alternator bracket. The other two (depending on the year) are copper braids from the rear of the passenger side head to the firewall, and from the front of the driver side head to the frame rail. These are often left off when engines are replaced or repaired, and then people can't figure out why their gas gauge reading changes when the lights are turned on, for example.

Do you have both of these grounds? If not, install them first. You can use 12 AWG wire rather than copper braid if you want. Here are where they go on most years.

[dhughins]

Sorry it's an 85. I know there is a big braided ground from engine to frame on passenger side. Gotta look for the rest. Thx.

[MalibuSSwagon]

85 would have a cable drive speedo originally, so unless it's been swapped for a later speedo, would not be related to an electrical problem.