Go to the current tire manufactures site, lookup your tires RPM (revolutions per mile). Then look for 20's with the same (or close). This is a bit more exact than simply looking at diameter.
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2011 Black Mustang GT Premium
Corsa XO & Sport AB/Airaid/Bama 93R
Best 1/4 mile to date 60' 2.009
12.538 ET @112.33
100+ Deg 85 Hum RPR 3PM 1/2 tank street tires