Regarding curb weights, Mercedes defines the listed curb weight with 90% fuel, one driver (68 kg) and luggage (7 kg)
"[5] data (filled curb weight with fuel tank 90 percent full, driver, 68 kg, and luggage, 7 kg) for curb weight according to Directive 92/21 / EC, version 95/48 / EC for vehicles with standard specification . Optional extras and accessories will generally increase as a rule this figure and reduce the payload capacity accordingly."
Take the listed curb weights and subtract the driver weight (68 kg / 150 lbs) and luggage weight (7 kg / 15.5 lbs) and the specs are:
C63 (90% fuel, 68kg driver, 7kg luggage): 1785 KG / 3935 lbs
C63 (90% fuel, no driver or luggage): 1710 KG / 3770 lbs
C63 S (90% fuel, 68kg driver, 7kg luggage): 1800 KG / 3968
C63 S (90% fuel, no driver or luggage): 1725 KG / 3803 lbs