When was the last time the car got a new thermostat? VW thermostats fail often. A properly working thermostat is paramount to good Fuel Economy. We burn fuel to make heat/pressure to drive the piston downward and drive the car down the road. A car that is chronically operating at 75C (which will still ready 190F on the dummy gauge) is going to consume more fuel than a car running at 90C like it should all other factors being equal.