I just bought a 1982 300sd and the owners manual says the fuel capacity is 18.5 gallons with a mention about a reserve.Does any one know the actual size of the fuel tank? Also is there a light on the dashboard that goes on when the fuel is low? I just filled up and gauge was near the letter R and it took 19.45 gallons.Is there a reserve amount when the gauge hits R ? It seems like the fuel tank is bigger then 18.5 gallons.

the reserve on the W123 is around 2.5-3 gallons... the owners booklet says the tank capacity is 21.5 gallons counting the reserve

there is a light near the gauge. triangular. it should light when you first turn the key. if not, check bulb.

if that works, but it doesnt come on when you are that low,(19.45 is a big fill) the contacts in the tank sender can get junked up.

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1984 123.193 372,xxx miles, room for Seven.

1999 Dodge Durango Cummins 4BTAA 47RE 5k lb 4x4 getting 25+mpgs, room for Seven.

I found out the hard way, I now fill up no lower than 1/4, I believe the SD tanks have a reserve of 2.3 gallons, I routinely get 400+ miles a tank.

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83 SD

84 CD