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on the toyota van i use for work, anything over 40mph reads 8mph higher on the speedo than it does on my tom tom, and my origin b2 which both show identical speeds
mine seems to be anything upto 5mph out. its worse at slower speeds, which is odd considering the faster you go the more a variance is likely to me magnified.
Neither.
Sat nav only works on the straight to be totally accurate the speedo about 5mph out so I just worked out what the true speed is by knokcing 5mph off the clock speeds.
After checking though the affects of tyre wear on the speedo (after all a smaller wheel near the limits doesn't travel as far per rpm as a new tyre) I'm not worried about it being out as I couldn't find any real difference.
My Origin B2 and Pioneer AVIC-X1 Sat Nav both read 4mph slower than the Speedo, but use the speedo anyway when going through speed traps just to make sure I'm not gonna get stung.