How big can I go with tires on my ‘09 ragtop GT?

[Kelman]

Saw 20” tires on a 05-09 era GT and wondered if there was issues involved? Also Id need to do something with speedometer, yes? Thank you & Happy New Year

 

[John Ainsley]

Welcome Kelman (I'm new here too). I understand you can use a tuner to adjust the speedometer when using a different tire profile. Hope some more expert chimes in.

 

[Svtpilot]

If you change the overall diameter of your rear tires it will affect your speedometer, as John said above most hand held tuners can correct the speedometer. However typically when some one upgrades to a larger diameter wheels, they use lower profile tires so the overall diameter may not change by much. For example a 2009 Mustang GT stock tires are 235/55-17 a 2010 Mustang GT with the Track Pack came with 255/40-19s. The 2010 wheels have a 2 inch larger diameter then the '09 wheels however because the '10 uses 40 series tires vs 55 on the '09 the overall diameter of the 19" tires are actually 2/10ths of an inch smaller then the 17" tires. 27.2" vs 27.0" That difference is not going to affect the speedo enough to worry about. As far as how big can you go? I have 305/35-19s on 19x11s in the rear and 285/35-19 on 19x10 up front, my car is lowered about an inch and I have no issues at all. With the correct offset and back spacing you could go a large a 305/30-20s in the rear without modifying or removing the bump stops. Both American Muscle and Late Model Restoration sell wheel and tires packages in various sizes and styles that are the correct size for your car. That is easiest way to get larger wheels and tires that will fit your car.

 

[mathews175]

I have 20's on my 08 GT all around from the guy who I bought it from. He said he kept the whole thing stock so I don't think a tuning is necessary, ive never had any problems.