Cleaning water jackets 225

Yes, I did install the sensor as suggested. Now, I'm unsure if my fuel gauge is working correctly as previously thought, here's why. The gauge read 1/2 tank and I drove 11miles to work and the gauge dropped to a 1/4 tank. I stopped and purchased fuel and it took 9 gallons to peg the needle past full. The car has a 18 gallon tank so I'm now thinking the gauges are off, since the gauge should have read 1/2. i will see if I can find out what's going on. The only thing I can think of is that I did change the altinator last season and maybe that's the issue. I gave them my old one and maybe this one isn't performing as it should.

Again thanks for reading and all the advice....

Regards
William