I don't have a 123 but my 87 w124 has never gotten less than 30mpg with a best of 34. The 30mpg was a tank full of 85 to 90mph drives though my 70mile one way commute. The 34 was doing 55 to 65mph depending on speed limit.

I recently started to record the mileage.

https://www.fuelly.com/driver/zorecati/300d?fu=5523902