Whenever I fill my tank I peg the needle about a 1/4" above full. It will take a good 50-70 miles for it to drop down to the full line. I'm usually about 100-130 miles into the tank by the time it reads 3/4's. At that point, the needle starts dropping faster and faster (I usually go about 200 miles on a tank, 230 will make it dry sump around almost any corner).

Could I have the wrong gauge in the car?

 

Mine's like that too. I believe you can actually get the fuel pump sender re-calibrated somewhere, but I don't remember where (maybe a DIY job? I don't know..)

 

first off the right gauge is the one thats used with 90ohms, second how did you wire it? 3 wires comin from behind the rear seats, black (fuel tank ground), redish/pink (12volts for fuel pump, so dont use this wire for anything with ur gauge), brown (sending unit)

 

I didn't wire it; the car came this way. I’m assuming it’s wired correctly or I probably wouldn’t be getting any reading at all. I thought that I might have a gauge for a different tank (different scale empty to full)? It’s the only thing I can think of unless the sending unit can somehow become uncelebrated.

 

Both my birds did it. The reason you get the sump around a corner is because the lack of baffles, so it seems like you have even less gas. The resistance in the tank due to the shape isn't a linear change so the needle reflects that.

 

I thought it might have something to do with tank design, but I didn't think it would be that drastic. Thanks for the reply firebirdjosh

 

Typical crappy GM sending unit. My 84' is the same, empty just below half. Gauge sky-rockets on full. My 86' chev half-ton is the same, on both of it's identical tanks.

 

Mines the same way sometimes, If i fill up while i have a half tank or so the needle will go way past full but if i run it till the needle is on E then when i fill it its just a hair above full...

Weird things, also i guess because GM was cheap they never calibrated the sending units for these tanks so 1/2 on the gauge is actually 1/3 of a tank