ok I have 15x8 steel wheels with 31 x10.5 tires, these things are heavy and as a result my MPG is a little better then 13. I was wondering how much I would be saving if I went back to the stock aluminum and got tire size 245 75 16 either the MTRs or some all terrain with wheel spacers of course. Should I even consider it?

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06 White KJ Sport 4x4
http://www.cardomain.com/ride/2432458/1

It's not really the wheels - it's the tires
My MTR's are 73 lbs on Aluminum Moab's and my stock Wheels/ tires were 45 lb's - wheel weight are almost identical and they cost 1-2 mpg
almost all of the tire weight is on the outside in the tread - it makes a longer lever that takes more torque to turn to get the wheel/tire moving or stopped
the wheel weight is concentrated in the center - even if it weighs more it doesn't take much more energy to turn (you will notice wheel weight in suspension response to bumps)

That said - if you have room to store a spare set of wheels/ tires - having a street set and an Off Road set is a good idea

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2005 CRD
stuff
Skeptic quod gratis asseritur, gratis negatur

Wheel weight would be more than 2 lbs. between 15 x8 steel and stock aluminum I would think, more like 10 lbs or so

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MUDD'S MOTORSPORT'S We do IFS lift installs ,
03 OVERLAND EDITION /Kilby-Skidrow-Mopar-4XGuard skids/ 2.5 inch TOTAL CONTROL JBA coilovers -JBA Arms/MOABS-31 FALKEN WILDPEAK AT3W / 4.10's etc, 04 Kilimanjaro Edition loaded, plus 05 KJ limited

My steel soft 8's weigh 27lbs a piece(16x7), my old steel rockcrawlers with the fake beadlocks weighed in at 38lbs(16x8) then add real heavy 265/75 MT/R's(55lbs apiece).