Right? 80Lbs more of just tires isn't insignificant, but no, not much of a change... maybe 1MPG?
So my truck, from the beginning, never got up to the stated MPG. Even 100% stock. I was able to get 21mpg on the expressway once (with the Baja Bosses and heavily laden with hundreds of pounds of fuel and gear for the Helene response when I did that). Otherwise, even when stock, I got maybe 14.3, 14.7 MPG. On long road trips before changing tires I got the same, without much difference city to highway. Right now, I'm getting about 13.7 city and highway. I haven't used Forscan to adjust for tire diameter yet and fix the speedometer- and maybe that'd have an effect, showing the true difference?