GPS speed accurate ?? - JKowners.com : Jeep Wrangler JK Forum
 
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post #1 of 11 Old 07-14-2009, 03:44 PM Thread Starter
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GPS speed accurate ??

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 ????

09 JKX 2in Old Man Emu lift and shocks, Teraflex 1in front spacers,35x16 Toyo Open Country M/T,16x8 eagle alloys,Raingler handles,XRC front and rear bumpers with carrier,Warn M8000 winch,6 in KC Slimlites,modified Rubi sliders. BROKE for now
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post #2 of 11 Old 07-14-2009, 03:47 PM
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They are usually pretty accurate.

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post #3 of 11 Old 07-14-2009, 04:00 PM
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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.

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post #4 of 11 Old 07-14-2009, 04:02 PM Thread Starter
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Quote:
Originally Posted by Jeep School View Post
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.
Yeah was in a hurry after the lift and I ahhh ahhh guestimated LOL

09 JKX 2in Old Man Emu lift and shocks, Teraflex 1in front spacers,35x16 Toyo Open Country M/T,16x8 eagle alloys,Raingler handles,XRC front and rear bumpers with carrier,Warn M8000 winch,6 in KC Slimlites,modified Rubi sliders. BROKE for now
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post #5 of 11 Old 07-14-2009, 04:05 PM
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I set my speedo with the procal to 31.25 with the Rubi wheels, And it's accurate. GPS is WAY accurate.

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post #6 of 11 Old 07-14-2009, 04:52 PM
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GPS is the best way to confirm after programming. If the speedo & gps don't match, start making adjustments until they do match.
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post #7 of 11 Old 07-14-2009, 06:46 PM
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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.

My GPS and Jeep speedo are D.N.O.

Last edited by chromedome; 07-14-2009 at 06:53 PM.
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post #8 of 11 Old 07-14-2009, 08:07 PM
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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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post #9 of 11 Old 07-15-2009, 08:42 AM
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My 35's are calibrated to 33.25. Dead on with the gps.

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post #10 of 11 Old 07-15-2009, 09:25 AM
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Quote:
Originally Posted by igofshn View Post
My 35's are calibrated to 33.25. Dead on with the gps.
same here
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post #11 of 11 Old 07-15-2009, 01:21 PM
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I agree with everyone else.

Trust the GPS and measure your tires properly.
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