Nobody regularly runs a 240D in the 30s unless they're selling a car on eBay.
Over 7,500 miles my '83 240D manual trans 4spd averaged 28mpg, one tank hit 32mpg but that was the highest I ever got in 30,000 miles.
Best mileage will be achieved by driving 60-65mph on the highway for hours at a time. Everytime you stop you're losing mileage...

Very common to have a thermostat that doesn't allow your car up to temp which will destroy mileage and carbon up the engine. Temp should be just over 80c indicated.