My fuel gauge has started acting abnormal. First time after 253K. It does not go past the full mark/line like it used to. Moves around more now

Seems to setting somewhat after about 15-miles after a fill- up

What wires, grounds, and or systems should I check first? All other electrical items perform normal. Thanks in advance to you who reply...
Patrick.
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'93 D-250 LE, Patriot Blue, reg-cab, auto, 3.54 LSD, BHAF, 3" straight-pipe/24" Wickedflow, Mag-Hytec diff-cover, ARE lid matches hood, 254K, 133rwhp/311rwt

, 18-26mpg.