I never got more than 31 in my 97 c230. If you're getting 40, you're measuring wrong. It's just not possible (unless you are running bicycle-width tires and added a 7th gear somewhere) Are you measuring Imperial MPG from Canada?

I can get 34 in my c350, with 7 gears, at no more than 60mph and with the AC off. That motor is turning 1800rpm at 60, and the 203 is far more aerodynamic than the 202. A 5spd 230 is turning 2500, and the 4spd 220 is turning at least 2800.

My answer (from 110k of driving one) is that under optimal conditions, my 97 5spd averaged high 20s with normal driving (70mph, AC) and typically around 23 in the city, with AC, and accelerating with traffic.

A 220 is 2nd-gear start, so that's going to help you some. You might nudge 25 in the city, under LIGHT load. You will be very lucky to crack 30 on the highway, especially on an older car (unless yours is in tip-top shape, new o2 sensors, MAF, filters and oil clean, etc)

__________________
former: 83 300D, 97 C230, 93 400E

current: 08 C300 Luxury , 92 500SL