I don't recall them affecting my mileage much. I was hoping they'd improve it but it pretty much stayed the same. That's checking it against the odo and the speedo is slow by 5 miles. My TC probably doesn't help any. And I don't hotrod it either.

 

I think 4X4 does make a difference. Sits taller to catch more wind, bigger tires (maybe?) and the extra weight of the front axle. How much a difference I don't know.