A taller tire will lower rpm, but at times that can also end up requiring more throttle to accelerate or maintain speed, especially with the brick-like aerodynamics of an ST, therefore hurting mpg. Lifting a truck to fit bigger tires will also hurt the aero. Late-model OHC engines like a little more rpm to be at their peak efficiency than an old-school OHV big V8.
Mid-70's 500-cube Cadillacs ran a 2.14 gear. :blink: That's why they're not worn out yet.
If you have 33" tires, seriously consider getting 4.56 gears to get the rpm vs. mph back to stock, it will probably help your mileage.