I borrowed a buddy's Garmin Nuvi 780 for my trip. I have the speedo recalibrated with my Pro-cal but the GPS still showed it being off +4mph. How accurate are the GPS's should I recalibrate to match the GPS ????
Not sure how you used the ProCal, but you should use your actual physical tire measurement, not what the stamp on the sidewall says. Plus, your air pressure can make a difference, too.
From everything I've read on the subject, the GPS worst case is off by .5 MPH typically though less than half that much.
I set my Procal by marking a tire and the ground at point "A". Roll the jeep in a straight line one revolution of the tire and mark point "B" where the mark on the tire lands. Measure this distance and divide by 3.14159 (Pi) enter that result into the Procal as close as you can.
I have a pre-decription GPS and the Speed is good to the tenth of a MPH. The GPS was my Speedo for years and CHP confimed it accurate. I Have the receipt to prove it. lol
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