Yes, those knobby off-road tires are good for nothing: Your mileage will be worse, they are noisy, need to be rotated all the time to keep from disintigrating, and they don't last very long. Yeah, if you really need to go off-road they help a bit..........help break stuff , by this time a person needs to get looking at an ATV to go "off-roading".

A 245, maybe even 265-70-17 should fill those wheel wells perfectly and you can get them in mileage-friendly tread designs. You may not even see any difference in mileage because these are still a fairly narrow tire.

It's when you go to the 31's mentioned or 285's and up that mileage will take a dump.