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275/60R20 speedo ?

Old 11-15-2013, 10:19 AM
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Originally Posted by RajunCajun84 View Post
I think you did it backwards... Larger diameter tire will rotate faster than the truck says. you will be going 72.5 when your truck says 70, if the difference would be 2.5mph like you say.

When I went from the stock 275/55/20 Scorps (approx 32" x 11") to the 33 x 12.5 my mph is dead on with a gps @ 80 mph. I did not compare the gps with the stockers because they were on the truck for 1 hr once I signed the paper.
Ha, yes thanks, well you are partially correct. Because of the larger diameter the mph will be off to the positive reading on your speedo. But the larger diameter rotates less actually. It has to according to physics.
http://m.discounttire.com/dtcs/infoTireMath.dos. Here is a great link for checking your tire size equations.

Last edited by F150_Feste; 11-15-2013 at 10:21 AM. Reason: Addtn
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Old 11-15-2013, 11:05 AM
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275/55 rev/mile 632
275/60 rev/mile 611

275/55 275/60 speedometer difference 60mph is 58mph -3.3% difference
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