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Old 07-14-2015, 03:13 PM
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Question - Speedo vs. Nav/GPS Speed

I just finished installing my MetalCloak Game Changer Fox Edition, 2.5" lift with Method 105 Beadlock wheels on 35" Trail Grapplers.

I have yet to use the FlashCal to adjust for tire size and TPMS, but when I drove it into work today I noticed the speed shown on the NAV screen was exactly the same as the speedometer, which I found odd...

Does the speed shown on the Nav screen go off of GPS or off of the vehicle speedometer?

I would think that going from the factory tires to the 35's it would certainly be off....

Thanks in advance and let me know!

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Old 07-14-2015, 04:24 PM   #2
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It would be off some. Download a speedometer app on your phone and that will tell you for sure. The way I do it is get on the highway and set my cruse to 60mph. Then I exit and adjust with the FlashCal. Then repeat until I get it pretty close. I think when I went to 35s I actually had it set to 33.5 inch tires on the FlashCal.

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Old 07-14-2015, 05:23 PM   #3
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When I ProCal'ed my rig (with 33's) I measured the radius from ground to hub center and doubled it. That number was within 1/2" of the final value that had me right on the mark and was closer than ProCal's recommendation to measure ground to top of tire.

With 33's my MPH value was off by about 3 MPH at 60 compared to a separate Garmin GPS.
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Old 07-16-2015, 12:59 PM   #4
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FWIW, my speedo was 5% off (Reading fast, at 70 it would read 73-74) with the stock BFG MTs. I just corrected it with my FlashCal by setting the tire size to 30.5" instead of 32".

So, I'm not sure what's up with Jeep's factory calibrations or the FlashCal's since both the factory setting and the FlashCal's 32" setting resulted in the same, wrong speed. I wouldn't trust the speedo even with stock tires, double-check with a GPS or GPS app on your smart phone.

I also verified using mile markers.
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Old 07-16-2015, 07:30 PM   #5
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Quote:
Originally Posted by Eary View Post
I just finished installing my MetalCloak Game Changer Fox Edition, 2.5" lift with Method 105 Beadlock wheels on 35" Trail Grapplers.

I have yet to use the FlashCal to adjust for tire size and TPMS, but when I drove it into work today I noticed the speed shown on the NAV screen was exactly the same as the speedometer, which I found odd...

Does the speed shown on the Nav screen go off of GPS or off of the vehicle speedometer?

I would think that going from the factory tires to the 35's it would certainly be off....

Thanks in advance and let me know!

I've seen the exact same thing with my factory NAV unit. Before and after I adjusted with flashcal NAV reads the same as speedo.
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Old 07-18-2015, 01:19 PM
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Yep. I hit it with the FlashCal for the 35's I put on and the speedo on dash and speedo on the nav screen still read the same. So I'm assuming the head unit gets its reading from the speedometer...
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Old 07-18-2015, 04:33 PM   #7
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Originally Posted by Eary View Post
Yep. I hit it with the FlashCal for the 35's I put on and the speedo on dash and speedo on the nav screen still read the same. So I'm assuming the head unit gets its reading from the speedometer...
As a related side note, does the stock 703N show the speed somewhere?

Thanks,

Flash
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Old 07-18-2015, 04:54 PM   #8
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Quote:
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Yep. I hit it with the FlashCal for the 35's I put on and the speedo on dash and speedo on the nav screen still read the same. So I'm assuming the head unit gets its reading from the speedometer...
You're correct. You would think it should be the other way around but it's not.

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