so your came stock with the 225/75 r15? 28.3 inch tire that is what mine had.
and u have a 31 .8 so more a less a 265/75 r16?
yeah it is 12% but get a gps or another car and have it do 60 and u follow and adjust gps works the best.
i went from 225/75 r 15 to a 30x 9.5 r15 which is 5.xx% but on my calubration i have a 2 in speed.
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