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I replaced the fuel tank (16 gal) and the sending unit in my 1968 fastback. For some reason it doesn't seem correct. Last week the needle was pointing to 1/4. I stop for gas and I put in just over 6 galleons, so there should be 10 galleons, between half way and 3/4 of a tank. Instead the needle jumps all the way to just under FULL. This might explain why it drops quickly, I drive it for about 15-20 miles and it will drop about 1/8 at a time. It is just an untouched 302 4 barrel with 130000 original miles as so I was told. I am almost certain the fuel guage is off, there is no way I am getting that bad mpg. Was there something I had to do to the sending unit before installation? The sending unit and fuel tank came together as a kit. I just don't want to take the chance one day and run out of fuel because I thought the fuel guage was off. Any suggestions?
 

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The same thing happened to my stang one day, the fuel sending unit just stopped working and it threw my check engine light on. luckily it fixed itself. you could fill up with fuel one day until it's totally full and measure your mpg to determine if the unit is off. but this would only work if you already know what your mpg should be though.
 

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The fuel gauge works by showing a resistance to ground that changes as the float raises or lowers in the tank.

Since the level is bouncing around, I agree...it sounds like the gauge or the CVR is bad. Or maybe someone tried to recalibrate the gauge and messed it up.

To eliminate the sending unit as the problem, remove the wire/boot from the post on the gas tank and ground it to the chassis using a short length of wire. The gauge should read full and remain steady. If it doesn't, I'd check the CVR. If that looks like it's putting out the right voltage, then I'd replace the gauge.
 
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