I'm just curious

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After changing from unstudded 205/55-16 Continental winter tyres to my 225/45-17 Michelin Primacy, I notice a slightly better MPG with my summer tyres. (from 48 to 50 on easy extra urban driving - computer display).
I also had to correct the toe-in slightly (was 9' out on both front wheels) which caused the car to "wander" a bit on our tarmac, which is worn by studded tyres. the 225-tyres are quite sensitive.