My guage "flickers" when my tank gets near 1/4 full. I understand that this is caused by a dirty fuel sender contact, but there is a separate switch to activate the "R" light. When my "R" light goes on, I'm down to a little over a gallon left in the tank, so I know it's time to fill up at that time. I keep a spreadsheet of my fuel consumption, so I have a feel for how long I can go on a tank, and I seldom am more than 20 miles from a station that sells diesel. Hope this helps,

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Robert W. Roe
1984 300SD 173K mi