I switched to 17" FJ Cruiser wheels (basically Trail Edition wheels) with OEM Dunlop tires and I didn't notice much difference in handling or gas milage. The TE/FJ wheel & tire combo is actually 5 lbs. lighter per wheel than the 20's and that seems to offset any increase in rolling resistance (in terms of gas mileage). If anything, the handling marginally better with the 17's so I'm happy with the change. I expect I'll notice more of a difference when I get around to buying new tires.
I went with the 17's because of the very limited selection of tires that fit the OEM 20's. The only all-season tires that fit 7" wide 20" rims (according to their manufacturers) are the two 245/60R20 OEM tires (Bridgestone Dueler & Yokohama Geolander), the
General Grabber HTS, and the
General Grabber UHP. Both Grabbers are 275/55R20 so they're slightly larger than stock. Continental used to list two 275/55R20 tires as fitting 7" rims but they recently changed their
spec sheet to 7.5". Bridgestone makes Blizzaks in 245/60R20 but I doubt you'd have much use for those in Alabama...
Anyway, I'd stick with the SR5 wheels and upgrade the tires if you still have stock ones, you'll have way more options and they'll cost less. Plenty of people run 275/55R20 tires specced for 7.5" or larger rims on the stock 20's with "no issues" but I was more comfortable sticking to official manufacturer guidance.
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Katahdin's 2012 Black Limited 4x4 – Automatic Running Boards / WeatherTech FloorLiners & Cargo Liner / 2012 FJ Cruiser Stock 17" Alloy Wheels & Tires / Blue Ridge Overland Gear MOLLE Seat-Back Panels / Ellis Precision "Black Ice" Shift Knob