[Jim Allen]

Well, I guess I get to bump this thread yet again by reporting that, on my '05 F-250 HD 4x4 (reg. cab, longbed, 8200# GVW) 5.4L automatic, with 4.10:1 ratios, going from a 245/70R-17D to a 285/70R-17D did not cause a drop in MPG. In fact, it gained a skosh (though it not really outside a reasonable margin for error). Yes, I recalibrated the speedo to match the exact loaded circumference of both tires (the factory calibration was within a few millimeters for the stock tires) and am using the PID feature of my Edge Programmer to monitor instant and average FE. Oddly enough, I also lifted the front end about 2.5 inches to clear the tires and that had no measurable effect on economy by itself ( I guess going from a garage door to a 2.5 inch higher garage door...). Combined with the tires, which had approximately 1.1 inch more radius, dropped my rpms by 200 and my effective gear ratio to about 3.84:1. The engine seemed to like it. No loss and perhaps up to a 0.25 mpg gain. Seems counter-intuitive... but I'm not complaining.

Bear in mind that my ultimate goal is not the highest mpg. What I seek is to make my truck more efficient configured as I need it (as a work/farm truck that takes an occasional trip).