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Old 02-22-2012, 08:08 PM   #1
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Speedo and Tire press - non stock tires

My new (new to me) 2008 Unlimited came to me with new Cooper Discoverer R/T3 LT285/70R17 tires instead of the stock P255/75R17 tires.

What range should I be looking at for the tire pressure? Stock tires were at 35psi.

Also, how does this affect my speedometer reading in MPH? Do any of you have your speedo adjusted when you change tire diameter?

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Old 02-22-2012, 09:41 PM   #2
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Originally Posted by pakmanpony View Post
My new (new to me) 2008 Unlimited came to me with new Cooper Discoverer R/T3 LT285/70R17 tires instead of the stock P255/75R17 tires.

What range should I be looking at for the tire pressure? Stock tires were at 35psi.

Also, how does this affect my speedometer reading in MPH? Do any of you have your speedo adjusted when you change tire diameter?
You should run 33's at around 30-32 PSI. A lot of tire shops, dealers, etc overinflate em, usually going by the door placard. The 30-32 is just a starting point. To find out the best PSI for any tire, use the chalk test.

There are two versions:

For the first, you simply draw a straight line across your tires, then drive on em and see how the line itself wears. You can use chalk, grease pencil, crayon, etc...just make sure it's a dry day. The line should be worn away evenly across the tread of your tires...if there's too much chalk left in the middle, the tire is overinflated. Too much left on the sides, underinflated.

There are some photos at the bottom of this link to illustrate what I mean:

http://www.jpmagazine.com/techarticl...psi/index.html


For the second, you need some sidewalk chalk and a flat asphalt parking lot or driveway. Here's a link:

http://www.ehow.com/how_6948382_chalk-tires.html


Your speedo accuracy won't be affected by the change in tire pressure; only if you change tire size.

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Old 02-22-2012, 09:50 PM   #3
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Thanks! I'll try the chalk test.

The Speedo question related to the change from the factory P255/75R17 tires to the current LT285/70R17 tires.
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Old 02-22-2012, 09:51 PM   #4
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Tire Dimensions Made Simple - Discount Tire

this link will tell you your speedometer difference
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Old 02-22-2012, 10:24 PM   #5
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Thanks! I'll try the chalk test.

The Speedo question related to the change from the factory P255/75R17 tires to the current LT285/70R17 tires.
Ah, gotcha.

The only way for YOU to adjust it would be to buy an AEV Procal, or a performance tuner like the Flashpaq Superchips.

It may or may not have been recalibrated by the previous owner. Sometimes dealerships and tire shops will flash the OBDII system for you when you get new tires; most of the time, they won't.

Easiest way to tell if your speedo is currently accurate or not would be to match it to a GPS that gives you mph, or maybe a GPS style download on your smart phone.
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Old 02-22-2012, 10:36 PM   #6
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Just downloaded a MPH app for my iPhone will try it out tomorrow! Thanks kbwwolf!

Thanks for the link whitejeep! I put in the specs from both tires and it looks like the speedo difference would range from .5 mph high at 25 mph to 1.42 mph at 70 mph. So if I stay about 1 mph low in our friendly neighborhood speed traps, I should be okay!
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Old 02-24-2012, 04:59 PM   #7
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Used the GPS app on my iPhone and came out with the basically the same results. 1-1.5 MPH high at highway speeds.
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Old 02-24-2012, 07:14 PM   #8
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Used the GPS app on my iPhone and came out with the basically the same results. 1-1.5 MPH high at highway speeds.
Sweet.

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