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I have anecdotal evidence from when I went from 195- 15 tire to a 215- 17 tire on my Jetta TDi. Sorry but can't recall the aspect ratios. The tires were very close to the same diameter, the larger tire and wheels being an option on higher trim models.
It absolutly had a negative effect on MPG. roughly 2 mpg. The larger tire has more weight and also more rolling resistance. Car did handle and look much better.
I would think there would be a measurable positive effect going to a smaller tire.
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