I replaced an after market gauge in my truck with an original dash. I hooked everything up and it all seems to be working, but I'm worried my gas gauge isn't functioning properly. When I start my truck, the gauge shoots past full and slowly creeps down to about 3/4 a tank. This means one two things. Either my something is very wrong, or I have 3/4 a tank of gas. Any input? I don't feel comfortable driving a classic truck not knowing how much fuel I have.

 

Well eliminate the easiest part first, drive to a gas station and fill it all the way up. If it still reads 3/4 tank of gas, then you know you have a problem.

The problem will either be electrical or a stuck float.

Overall though, I'm betting filling it won't change things, and it doesn't sound like a stuck float, so I'm predicting electrical issue.

 

On the electrical side, try grounding the sending unit wire with the key in "run" - if this makes the gauge go all the way past full, then the issue is probably that the sending unit is set up for a different resistance than stock, for that aftermarket gauge. (The gauge has ~5V to it from the IPVR, and grounds through the sending unit, which has variable resistance - lower resistance when full).

 

X2 on what he said...and a little extra.

Fuel gauge issues?
Disconnect the wire that goes to the fuel sending unit at the tank. Ground that wire to the frame, making sure the frame is clean. Turn the ignition to the "run" position and watch the fuel gauge. If the gauge swings all the way to full, either the sending unit itself is bad or it has a bad ground. If the gauge stays on empty then either the gauge is bad or there is a wiring problem.
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To pop the plug off for the test, you can barely get up in there with your head and see the plug. While looking at it, VERY CAREFULLY use a screwdriver to pry it off. AND I MEAN VERY CAREFULLY. It is only pushed on like a 1/8". This is only if you can’t get it with your hands.
Once you have done the test (above) and decide it’s the sending unit. And you have cleaned the ground and that did not work. Then disconnect the ground again and fuel line and read on….
If you can't get either one disconnected, no worries you will have a little more room once the tank is dropped down a little. But make sure you have some wire slack in the plug connector.
Run the tank dry or almost or siphon it empty, I would not do it with ½ a tank of fuel, but if you just have to, just use a big floor jack and a piece of 2x2 plywood. It’s not heavy at all, unless you have it full or 1/2 full of gas. SMART THING TO DO IS DRAIN THE TANK!
Loosen the filler and vent hose clamps at the tank end and maybe you can pop the lines off? But most of the time you have to just loosen the clamps and wait till the tank is about out.
With the jack in place, or not, just loosen the hdwr (2 bolts) on the front straps, and remove the tank straps aft hdwr all together, then swing the straps out of your way.
Slide the tank to the dvrs side about 1/2" and then pull out or let the passenger side come out of the frame. Then disconnect the elec connector and eng feed fuel line if you could not get to them earlier.
Then slide it as far as you can to the right and pop off the filler/vent lines. Sometime you need to carefully put a flat tip screwdriver in between the rubber hose and the tank to break it loose. The filler neck and vent lines get stuck to the metal pretty good sometimes.
Once the tank is removed there is a lock ring that comes off with a screwdriver or punch, look for a small tab that has been bent over to lock it. Un-bend it, tap off the lock ring, remove the sending unit.
Insp the underside of the tank where the support straps rest, lots of dirt and moisture build up there, and that causes the bottom side corners to rust out. You can also come reinstall time replace the anti-chaff material. Old 2" ratchet strap work great, glued to the support straps. (Rub/coat them with grease so they do not retain moisture, or coat them in the plastic tool handle dip). Or an old piece of inner tube that way it is moisture resistant.
Insp the inside of the tank, might be a great time to replace it all together? Before you reinstall anything connect the electric connection and ground to the sending unit. Wire it up under the truck so it is not hanging by the connection. And with the key in the 1st (on position) you should be able to move the float arm and see a difference in your gauge?? It is always easier to do this with a helper.....
Also before install, paint the outside of the tank with some good paint. Even spray on bed liner or undercoating??