2646 is much too much of a change for a 2 mph error (which is about 2-3%).
You did not say if the error was fast or slow. What I'm asking here is is the speedometer saying you are faster than the GPS, or is it saying you are slower? And, are we talking about the dashboard speedometer or the Gryphon speedometer?
If the Gryphon speed was 2 mph slow, try 2533mm. If the Gryphon speed was 2 mph fast, try 2433mm.
But if the dash speed was 2 mph fast, you were probably dead on.
And, according to your FIRST post in this thread, your tires were 275/65-18s. Now they've "morphed" into 275/70-18s? The 70-series tires are a bit over 1 inch larger than the 65s.
- Jack
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