The I6 in question is the well renowned 4.9L, yes?
Otherwise, 35" tires with 4.56gears will ruin mileage - guaranteed. With the right aftermarket transmission ( WELCOME TO RICHMOND GEAR
) you might only lose 5-10% in fuel economy. With the greater expense to modify the vehicle to get better mileage with the lift, gears and tires won't show any significant benefit. I bought my 'truck' (SUV in disguise) for what it can do - tow, haul and guzzle gas in the process.