I have owned a 2001 W210 E240 which in reality had a 2.6 V6, A 2004 CLK 240 which had the same engine and now have a 1989 Japanese spec W124 260e

I never have reset the mileage computer on either of the W210 E Class or W209 CLK.

Results E240 for 34,000 Kilometres = 10.2 litres per hundred K's which is around 24 mpg - combined city/country driving
Results CLK 240 for 38,000 kilometres = 9.8 litres per hundred kilometres which is around 25 mpg - combined city/country driving.

No computer in W124 260e but after about 15 tanks averaged about 13 kilometers per litre which is around 18mpg - combined city/country driving

MPG figures are for U.S. gallons not imperial gallons.