The bigger tires do not help, extra weight and rolling resistance, not to mention the extra diameter puts extra strain on the engine due to the lower effective ratio. My 2nd gen X averages about 12-14 mpg with 33" tires, titan swap, all the fun parts like skids and RTT. SoCal terrain is not friendly to MPG either, I averaged 16-18 mpg on my trip back to the east coast once the land flattened out.