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.