Just found this on another forum,,funny...
I used to work on a fleet of Mercs 307/308/814 and 1617s
The 307s were very slow due to having a large box body and tailift but brilliant milage munchers we had one with 450K on the clock when it got sold on.
The MD was a proper control freak and fitted the black box spy thingys to all the vehicle. When the stats got printed out they were shocked that the 307Ds were doing less to the gallon that the 814s (16ish for for the and 17+ for the 814s). Further investigation found that most of the drivers reckoned the vans went quicker in 4th than 5th gear.