Ok as many of you know I am not super good at this stuff, when do you really need to replace the tires? I have 20k miles on my X5 and I'm unsure when I should replace them?

Its a 3.0i with sport pkg ('05 (I think)

I also was wondering how much they cost?

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The Present:
2014 Audi Q5 TDI Prestige
The Past:
2013 Lexus GS350
2013 VW Golf TDI
2007 BMW X5 4.8i LOADED & Loved
2009 VW Jetta
2008 VW Touareg VR6
2005 BMW X5 3.0i
2005 BMW Z4 3.0i
2004 BMW X5 3.0i
2003 BMW 325i
2000 Ford Explorer Eddie Bauer 4x4