Car appreciation... the aesthetics, the engineering, etc

Got a long trip coming up and wondered if there was some boost in highway mileage From using RON97.

If the ECU can advance the ignition timing more with the higher octane fuel then you might end up with more efficient combustion and so higher mpg. Against that you've got to offset the lower specific calorific value of the higher octane fuel and, of course, the increased price. Then there's the complicating factor that, IIRC, the 1.5 TSI uses a Miller combustion cycle at some points in the engine map, which will tend to reduce knock and might mean that there's no benefit in using higher octane fuel since the ignition timing will be optimised already with 95RON stuff.

So that's a definite maybe from me. Only way to find out is to test it, I suspect. Like danb says, the engine will need time to adapt to any change of fuel.