Hm. I wonder if mine has been replaced before because my gauge is like.. when its on full the needle goes to 3/4 then when after like one drive or two it goes 'full'
once it really gets to 3/4 it stays for a long time then its like suddenly little less then half and goes normal from there lol
It's probably just stuck on the hinge or other contact point. Taking the unit out and cleaning the hinge/contact would help. Sometimes it gunks up, at least that was the case on other cars I've dealt with.