It looks like Muscheldiagramm is called a BSFC chart in the USA.
It stands for Brake Specific Fuel Consumption.
Those are ideal to have to help one find the sweet spots in different gears and speeds.
Old Tele Man has great advice regarding the rule of thumb if one doesn't have a copy of a BSFC chart for their vehicle. Soemtimes they are hard to find.
http://ecomodder.com/forum/showthrea...html#post14985
Brake Specific Fuel Consumption (BSFC) Maps - EcoModder