I've seen a 6psi change in tire pressure affect mileage to the tune of 3-5 miles (all things roughly equal). Doesn't suprise me that adding to the sectional width, rolling resistance, and moving from LRR to non at the same time would mean a 10% change.
I've got non-LRR tires, and am considering going back to LRR's just to eek out another 5-7 miles of range. That's alot in these cars!