35s there really is no need to regear I am getting on average 550-600 a tank on a 23gal cell best I have seen was 630 when driving from seattle to so cal back in june

Check that fuel tank size. I believe the mfg. rating for the tank is 29gal. But you can pack in another 3gal. By your calculations you're getting 23-26mpg. 26mpg is the highest fuel milage I have heard of a powerstroke getting....no lift, 2x4, street tires, accelerating to 65 takes a mile, braking takes a mile, not exceeding 2000rpm...etc.

Have you compared your spedometer to a GPS? My tires throw me off +5mph. And at highway speeds, for every 50mi I drive my odometer registers +40mi. These are the size tires that came on the truck. Fixing to change them out and go down in size. Our factory gearing is 3.73. If your spedometer is right, then the PO regeared or added an electronic speed calibrator. Yes, 35's is on the outside edge of the grey area for regearing. You're truck takes a lot more effort than a Cheve 1500 4x4 to get going. Harder acclerations = higher trans temps and less fuel milage. The 7.3 trans cooler is undersized for the job a 7.3 is rated for. A lot of 7.3 owners swap it out for a 6.0 trans cooler. The toque converter just covers stock 7.3 output as well. It's usually the first thing to go when the trans is pushed to its limits. I belive there is still a powerstrokehelp.com video where Bill (of powerstrokehelp.com....lot of Bills out there) goes over a truck with 35's that he is replacing the torque converter for the 2nd time.