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While both are rated as LRR, they are apparently not the same.
(make + model + rubber compound, yet even then there are differences)
The wider tyres cause more aero drag.
Heavier wheels+tyres cost you when accelerating - even if you get some of that back when coasting.
The circumference is 0.6% bigger on the 17" - so it's slightly under-registering the miles driven - you might want to compensate for that, but it's not enough to cause a 4-6mpg difference.
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Strayed to the Dark Diesel Side
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