Q: Gas gauge needle fluctuates up and down

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We just bought this car yesterday, it has 110,000 miles but was checked out by our mechanic and he gave us an excellent report. We had to put fuel in so we filled it up and drove 6 miles home. In that short period the needle fluctuated from full-over full to 3/4 a tank! I know these cars are supposed to get good gas mileage so what would cause the needle to do this? We are still within our 72 hours return so I am looking for a quick answer. Thank you!!!!
My car has a manual transmission.

This may possibly be a faulty fuel tank sending unit. This is a float type designed sending unit that sits inside your fuel tank that monitors the fuel level inside the tank. When this is faulty or failing it can cause intermittent changes in the fuel gauge to register as you describe.

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