99 Suburban-Inaccurate fuel guage?

It appears that my fuel guage does not seem to be registering properly.One day the wife said there was between 1/4 and 1/2 in the tank.I took the vehicle to fuel it up and while driving,the guage dropped into the red area near empty.When I fueled up.I put 163 liters of fuel in the tank.The owners manual say the tank holds 167.I cut that pretty close.Since then I have been using the trip odometer to help me decide when to fuel up.The guage does not fluctuate wildly but it appears it does not travel below 1/2 a tank.I have monitored the amounts that I put in to see if the guage could be working properly,but I don't really trust it.I'm thinking that the sending unit may be compromised, but I am looking for other suggestions to help correct my problem.I do add a product called Megalene to help keep the fuel system clean.I add 10 ml. of Megalene for every 40 liters of fuel.Could this product affect the fuel sender?I've used this product for a few years and the guage seems to be acting up in the last six months.Thanks for the help.

chuck's

Replace the sender in the tank and look at the condition of the windings, that will tell you if your additive has affected it.

My '94 Sub also does this; one has to drop the fuel tank to replace the sender. We use the trip odometer instead.