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Unread 12-07-2011, 07:37 PM   #1
jtjohnston
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calibrate speedometer?

I have just put on 16" snow tires, down from the 17" rim - on my new 2008 JK Wrangler (new for me).

Is my speedometer still working correctly? It seems like I'm driving awfully slow at 100 kph?

Do I need to go to the dealer to have the computer adjusted?

I lost 3 inches of height! Makes jeepy look awfully petite!

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Unread 12-07-2011, 07:40 PM   #2
DrHolliday
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rim size has nothing to do with it, need to know the tire sizes... An AEV procal can correct for just about any tire size.
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Unread 12-07-2011, 08:00 PM   #3
jtjohnston
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Why would I change the axle ratio?
http://www.aev-conversions.com/pdf/i..._Reference.pdf
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Unread 12-07-2011, 08:05 PM   #4
spyder6
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Quote:
Originally Posted by jtjohnston View Post
for several reasons, but its not what you need.

what the pro cal does is change the ratios available. your constant variables are your rear end ratio and the output speed of the driveshaft. what you've changed is the outer diameter of the tire, which is also a variable to the whole system.

you need to calibrate the computer, with a pro-cal or other similar device. that way the computer will know the change in tire height and the speedometer will work accordingly.
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Unread 12-07-2011, 09:36 PM   #5
BManz
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You can play with this Excel file to quickly see speedometer changes if you change tire diameter or axle ratios:
http://www.jeepforum.com/forum/f96/s...lator-1298121/


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Unread 12-08-2011, 06:31 AM   #6
aristobrat
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Quote:
Originally Posted by jtjohnston View Post
I have just put on 16" snow tires, down from the 17" rim - on my new 2008 JK Wrangler (new for me).

Is my speedometer still working correctly? It seems like I'm driving awfully slow at 100 kph?
To recap what others have said, if the overall size of your tire changed any (32" to 31", etc), then your speedometer will not be working correctly.

If you want to verify your speedometer, use a GPS that shows your current speed, and compare it to your speedometer. If you don't have one, or can't borrow one, a lot of smartphones have free apps that do that too.

If the overall size of your tire did change, to recalibrate your speedometer, you can have your dealer flash the new tire size in (and continue to pay your dealer each time you swap tires), or you can pay for a ProCal once, and make the change yourself when needed.

The second page of the ProCal PDF you uploaded shows how to change tire sizes. Oh, and you need to measure the actual physical size of the tires on your Jeep. Do not go by the size stamped on the side of the tire, as that's often not accurate.
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Unread 12-08-2011, 06:33 AM   #7
WXman
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I've had my JKU for 6 months and I've been through 5 different wheel/tire combos already. My AEV ProCal has paid for itself 3 times already. I love it.
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Unread 12-08-2011, 03:39 PM   #8
jrmandryck
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Ditto all about the overall rim/tire combo size
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Unread 12-09-2011, 07:48 AM   #9
WXman
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Just FYI.. what I am discovering is that with my ProCal, I have to set the ProCal to .5" LESS than the actual measured tire height while on the Jeep. This is the only way I can get my speedo to match up with GPS readings. It was this way with both my 32s and 33s.
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Unread 12-09-2011, 07:58 AM   #10
Noxian
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My understanding is the dealership can make adjustments for tires up to 32inchs also.

But I recommend the Procal. Love mine, lots of other cool stuff it can do too.
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