What should the gas mileage be, per tank and per gallon, for a 1990
Mercedes 300TE, in good working condition?

Dear berniemarlene,

According to the Fuel Economy Database of the U.S. Department of
Energe, you should expect the following gas-mileage-values for a 1990
Mercedes 300TE with correctly adjusted engine:

City traffic:     17 miles per gallon premium gas

Highway traffic:  20 miles per gallon premium gas

Combined average: 18 miles per gallon premium gas

Since the fuel tank of a 1990 Mercedes 300TE has a capacity of 19
gallons, you should be able to drive the following distances with one
tank:

City traffic:     323 miles per tank filling premium gas

Highway traffic:  380 miles per tank filling premium gas

Combined average: 342 miles per tank filling premium gas

The 300TE 4matic, however, has slightly different data concerning gas
milage due to different gas consumption figures in city traffic:

City traffic:     16 miles per gallon premium gas

Highway traffic:  20 miles per gallon premium gas

Combined average: 18 miles per gallon premium gas

This leads to the following average driving distances per tank
filling:

City traffic:     304 miles per tank filling premium gas

Highway traffic:  380 miles per tank filling premium gas

Combined average: 342 miles per tank filling premium gas

Sources:

Fuel Economy, by the U.S. Department of Energy, 2002
http://www.fueleconomy.gov/

Gas Mileage of 1990 Mercedes-Benz 300TE, by the U.S. Department of
Energy, 2002
http://www.fueleconomy.gov/feg/noframes/6956.shtml

Gas Mileage of 1990 Mercedes-Benz 300TE 4Matic, by the U.S. Department
of Energy, 2002
http://www.fueleconomy.gov/feg/noframes/6957.shtml

Edmunds.com: 1990 Mercedes-Benz 300TE 4matic performance datasheet
http://www.edmunds.com/used/1990/mercedesbenz/300/4dr300te4wdwagon/specs.html

Edmunds.com: 1990 Mercedes-Benz 300TE performance datasheet
http://www.edmunds.com/used/1990/mercedesbenz/300/4dr300tewagon/specs.html?id=lin0066

Search terms used:
Mercedes 300TE 1990 "gas mileage":
://www.google.de/search?hl=de&ie=UTF-8&oe=UTF-8&newwindow=1&q=Mercedes+300TE+1990+%22gas+mileage%22&meta=
mercedes 300te 1990  "fuel tank":
://www.google.de/search?q=mercedes+300te+1990++%22fuel+tank%22&hl=de&lr=&ie=UTF-8&oe=UTF-8&newwindow=1&start=20&sa=N

Hope this answers your question!
Regards,
Scriptor

A great answer.  No comment to add except to say that I am very satisfied.