In my case, I find the reserve light to be very accurate on my 124 cars. If I fill the tank as soon as that light is on steady, my '87 wagon takes 16.5 gallons, my '95 sedan takes 20 gallons. I've found this to be very consistent.

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Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC