I put a new fuel tank in my 69 roadrunner. The fuel gauge does not work properly. It didn't work right since I got the car back 13 years ago. when the tank is full it only reads about 1/4 on the gauge. I have put a new sending unit in it twice and still it doesn't work like it should. It reads the same. There have been three gauges in it since I got it back and it always reads the same. Is it a bad gauge in the dash or maybe a bad ground.