Your C280 has, I believe the 2.87 final drive. Later five speed cars have the 3.07 ratio.
At 2.87 and the 195/65-15 tires, you would be spinning 840 times per mile. So, at 60mph, your engine is spinning 2400rpm.
At 85mph, you should be at 3400 or so. Keep in mind MB tachs usually read 100-200rpm high. So, at 85 mph, you would probably see an indicated 3500-3600rpm.
1998 C230 "Black Betty"