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Discussion Starter · #1 ·
I recently went from stock rubicon tires on my 2011 JKUR to 315/70/17 Mastercraft MXT. I used spacers with my stock rims. I also did a 2.5 inch Teraflex coil spring lift. Because of the lock pick, my speed would only show 0 mph. Anyway, since the tire change, I was under the assumption I was going about five miles faster than my speedometer was showing on the interstate. I just turned off the lock pick features. I drove it yesterday using the navigation to compare and it's EXACTLY the same. I drove at different speeds from zero to about 75 and it's spot on the whole time. How can this be!? What am I missing? The Nav reads from GPS, right, and not my speedometer...? On a side note, the distance traveled is off. It's basically reading .9 miles on my trip gauge versus 1 mile on nav.
 

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I would try it with a phone navigation that displays your speed. I think the only way your speedo is correct is via a ProCal or similar OBDII device. I'm not positive on where the nav pulls the speed from, but I believe it's from the vehicles speed sensor.
 

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Measure with an external GPS device and you should see the difference, unless you bought the Jeep used, and the previous owner calibrated for 35's, sold the vehicle, and the new dealer simply ended up putting the smaller tires, and you never knew about it until now :)
 

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If your stock Rubicon tires had an aspect ration of 75, then the new 70 isn't that far off. This is the number you are concerned about. So it would make sense your distance is off a smidge. Also, your mph should be slower than what is shown since your tire height has gotten shorter (again, assuming the Rubi tires were 75).
If you bought it used, then maybe what rsmwrangler says could be true also.
 

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Discussion Starter · #5 ·
Good idea, I'll try with an external gps to see what happens. I bought the jeep new and it did have the stock rubicon tires, 255/75/17. If the speed ends up being accurate again (not that I'm complaining, just confused!) would a procal or anything else really benefit shifting points or would those be the same too?
 

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If your stock Rubicon tires had an aspect ration of 75, then the new 70 isn't that far off. This is the number you are concerned about.
Okay, that's just... not right at all. The 70/75 is the ratio of sidewall to to tread and has nothing to do with whether or not the speedo will be accurate.

315/70R17 means that the tire is 315mm wide, the sidewall is 70% of that (220.5mm or ~8.7") and it fits on a 17" wheel. 17" of wheel + 8.7" on top + 8.7" on bottom = 34.4" tires.

255/75R17 ... 255mm wide, 75% sidewall (191mm or 7.5")... 17" of wheel + 7.5" on top and 7.5" on bottom = 32" tires.

You can go even smaller... 215/70R17... 6" of sidewall - 29" tire.

All that aside - your speedo difference will increase as your speed increases. At 70MPH indicated, you'll actually be traveling ~75mph. You can use this link to pop in tire sizes and see what your indicated vs actual speed will be.
 

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You need to check your speed not using the on board GPS. It is linked. With an auto trans. I think you would benefit using a Procal.
 

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Something is not right here.

With my stock 255/75-17'son my JKU HR the speedometer indicated 1 MPH fast compared to my handheld GPS.

After replacing the tirs with 285/70-17's the speedometer now reads 1 MPH slow compared to GPS.

With you much larger (in diameter - which is what matters) 315/70-17's your speedometer should be reading 4-5 MPH slow compared to actual speed. A great way to get speeding tickets.
 

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Discussion Starter · #9 ·
Okay, that's just... not right at all. The 70/75 is the ratio of sidewall to to tread and has nothing to do with whether or not the speedo will be accurate.

315/70R17 means that the tire is 315mm wide, the sidewall is 70% of that (220.5mm or ~8.7") and it fits on a 17" wheel. 17" of wheel + 8.7" on top + 8.7" on bottom = 34.4" tires.

255/75R17 ... 255mm wide, 75% sidewall (191mm or 7.5")... 17" of wheel + 7.5" on top and 7.5" on bottom = 32" tires.

You can go even smaller... 215/70R17... 6" of sidewall - 29" tire.

All that aside - your speedo difference will increase as your speed increases. At 70MPH indicated, you'll actually be traveling ~75mph. You can use this link to pop in tire sizes and see what your indicated vs actual speed will be.
That's what I thought too, reading 65 would basically be 70, 70 would be 75, etc, at highway speeds. I don't suppose the gearing would have anything to do with it. Mine came with 4:10's which was included back in 2011, instead of the extra $700 or whatever it costs now to upgrade.
 

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That's what I thought too, reading 65 would basically be 70, 70 would be 75, etc, at highway speeds. I don't suppose the gearing would have anything to do with it. Mine came with 4:10's which was included back in 2011, instead of the extra $700 or whatever it costs now to upgrade.

Something like that, but it's uh... logarithmic I think. 20 = 21, 30 = ~32, 40 = ~43, 50 = ~54

Those are very rough numbers but as the speed goes up the difference will increase.

And, as tires get bigger, that gap will increase quicker. So if you put 40's on, you might find that 20 = 23, 30 = 36, 40 = 52, etc.
 

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Something like that, but it's uh... logarithmic I think. 20 = 21, 30 = ~32, 40 = ~43, 50 = ~54

Those are very rough numbers but as the speed goes up the difference will increase.

And, as tires get bigger, that gap will increase quicker. So if you put 40's on, you might find that 20 = 23, 30 = 36, 40 = 52, etc.
This is how mine was when I swapped tires. Before reprogramming, the faster I drove, the more the speedo vs gps was off.
 

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Are 35" tires really, exactly, 35.0000" in diameter when installed?
 

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Are 35" tires really, exactly, 35.0000" in diameter when installed?
no. The tire manufacture states tire size of unmounted and then tend to round up slightly. They cannot take in account every vehicle and the variances of weight added to them ect. Most 35s will net around 33.5 once mounted.
 

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The navigation gets it's speed from the Jeep, not GPS (kinda lame). So your speedometer does need to be corrected. As mentioned by others, use an alternate GPS to see your actual speed.
 
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