I was surfin for tires for the FJ and found this Tire size calculator . It's on a miata site but it allows you overlay the tires to show the size difference and speedo error.
I was looking at 305/70R16 vs 285/75R16 and the calculator showed me that they basically the same except for width.
__________________ 08 Auto, Sandstorm, Conv Pack, Offroad Pack, Roof Rack, Tow Hitch, Toytec OME 3" Rear Scuba Mod
255 85 R16 BFGMT's Demello Winch Bumper on its way
SF Pre Runner - Flowmaster Series 50, Razors with BFG KO's, R.E.D Light Bar, PIAA 520's and PIAA 510 Fogs, AF HL Covers, Hella Dual Horns, Dirty Parts Dual Battery, aFe CAI, Vent Visors, Trail Gear Sliders, Demello Skid, AP 2" FOX Adj Res. Suspension, upper and lower Links, Shock Skids, Sway Bar Links, AP Airdam/light bar w/PIAA 510's, Coleman Rack Basket, Hi Lift, Spidertrax spacers, Tuffy Cargo Box and Ellis Interior Grab Handles.
08 Auto, Sandstorm, Conv Pack, Offroad Pack, Roof Rack, Tow Hitch,
I'm so confused 
Linear Mode
