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Most 35's are not true 35's. They are more like the 34.1-34.5 range, which in his case would be close enough. The only tires that measure what they say they are are interco tires.
Sorry it took so long to reply, I had my work schedule change and it completly ruins my ability to get in the forum. There was no change to the best of my knowledge. I took it in to the dealer about two weeks prior to towing my camping with the complaint of the mileage and they said nothing was wrong. According to the dealer all they did was hook it up to the diagnostic tool and made sure everything was working correctly.
Two weeks later I was getting better mileage. Maybe the change from winter blend or the outside temperature who knows. I had a 2004 6.0L and it was just the opposite, 19MPG when I bought it and then the MPG started going down until I hit about 25K then started to climb back up. I love my truck but it does seem odd how each one works a little different.