18 gal tank doesn't hold 18 gal

My older Maxima fuel tank is supposedly 18 gal capacity. I have not let it get lower than 1/4 tank remaining, but 9 gal will fill it up when it is at that point on the fuel gauge. I don’t get it. What’s up with that?

The simplest explanation is that when your gauge shows 1/4 tank remaining, you still have half a tank of gas. I wouldn’t be surprised if you could drive another 200 miles with that ‘quarter’ tank.

it can be a dented fuel tank or a fuel guage sender problem or the guage is broken

Few fuel gauges are anywhere close to accurate. They generally will show full if the tank is full and will show empty when the tank is empty, but anything else is anyone’s guess. Some are close, but many are way off.