hey Guri,
Im gonna say my mileage definitely was affected and i did it all about the same time so tough to say what had a larger affect. When my tires were fresh it was also a bit better, now they are at the end of their life and not super smooth.
Driving mostly on surface streets (at 8K' elev.) i am averaging around 15-16mpg. Highway is 16-18mpg and a little better for both if i am at sea level. this is with a consciously light foot, if i was to mash around who knows how bad it gets.
Fully overloaded with all gear and camper out on the trails i am around 10-14 mpg.
Im positive if i ran a more highway style tire i would have much better results, but thats the price we pay.
Im thinking im going to bump up a tire size this round and see how i like it. 34-35" range.