I assumed that when i went to the 285s my mileage would either stay about the same or improve slightly on the highway... WRONG. I lost 2 MPG at the same highway speed. Its worse if I cruise at the same RPM that I did previously (1950), I did pick up about 5mph at that RPM but it sure didnt help mileage. I bought the smarty jr at the same time to adjust the speedo, and after 2 fillups I went from the stock software to the econo tune to see if it would help... nope no difference. These tires are heavy, but I didnt think they were that heavy. Maybe they filled them with sand instead of air when I had them installed. :-laf
I do like the look though, they fill the opening perfect.
From Toyos website, the 285/70/17 E weighs 53#, while the 265/70/17 E weighs 47#. I can't imagine that 24# of rolling weight could reduce mileage THAT much... was there ANYTHING else you did to your truck at the same time??
PS--just because I can't imagine it, doesn't mean it's no true, so please know that I am NOT trying to say your wrong, just wondering if maybe other factors could be at play.