I often use the previously described 60/60 method (60 MPH for 60 seconds) to see just how close I am with tire size. It work really well for those who don't have a GPS. For more accuracy, set the cruise at 60 and do 5 miles vs. 300 seconds. This will get you REALLY close.
And as Jack said, the speedo is nearly always 2 MPH faster than the actual vehicle speed so even if you're 1-2 MPH off, you should still be safe.
Take care.
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Power Hungry Performance - The ORIGINAL in Ford performance tuning... Since 1996!
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