Why? The factory size (for supercoupes) is 225/60/16. A 225/50/16 is 24.9" in diameter while a 225/60/16 is 26.6" in diameter. a difference of 6.7%. Your speedo would be off by 4 mph at 60 (it'd read 56 while you were going 60)
If you want 225's go with the 225/60's.
1997 Thunderbird Sport
destined to remain stock
1997 T-Bird GT 4.6
80 MM Mustang GT MAF - P&P'd Mustang GT TB and Intake Plenum - PI Intake manifold - PI Heads
Mark VIII Torque Converter - J-Modded 4R70W transmission - Mark VIII Aluminum Driveshaft - 3.73 gears
PBR Brake Calipers - Eibach Springs - KYB Shocks
'89 SuperCoupe front and rear sway bars
SCT Chip programmed by Lonnie Doll
255/50/16 Falken Ziex ZE-512's
True Dual exhaust with Magnaflow DI/DO muffler
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For 16's id recommend 235/60/16's. They give a little extra height to fill up the stock fenderwell and have a little bit more width too, and my speedo seems to still be dead on or at least it's close enough for government work...