Forum Index
Jeep Horizons HomeLinksJeep Horizons Tech Write-upsJeep newsLand useContact

 Watched TopicsWatched Topics   FAQFAQ   SearchSearch   MemberlistMemberlist   UsergroupsUsergroups   RegisterRegister 
 ProfileProfile   Log in to check your private messagesLog in to check your private messages   Log inLog in 

New tires

Post new topic   Reply to topic     Forum Index -> Suspension, Steering & Tires
View previous topic :: View next topic  
Author Message

Joined: 07 Dec 2005

PostPosted: Wed Dec 07, 2005 9:01 pm    Post subject: New tires Reply with quote

Hi, a few months ago I bought some 20 inch rims for my 2000 Jeep Grand Cherokee that came with 27/45/20 tires. I was wondering if anyone knew if 255/35/20 tires would fit because I want to get lower profile tires.
Back to top
View user's profile Send private message

Joined: 17 Aug 2005
Location: Atlanta/Athens, Georgia

PostPosted: Wed Dec 07, 2005 10:07 pm    Post subject: tire sizes Reply with quote

20 inch rims aren't really my taste- I've got 16's now and I wish I had 15's because I would have a wider variety of tire designs and sizes (and they would be cheaper). First you need to know what size rim and tire came on the jeep from the factory. Did it come with 20's from the factory, were they a dealer installed option, or did you just go out and buy them? If you don't already know here is what the tire size means:

using the 245/75R16 as an example
the first number 245 is the width of the tire in millimeters
the second number 75 is the aspect ratio meaning the profile of the tire is 75% of the width (245) which equals 183.75
the third number is the rim size 16
to get the overall diameter you would add 183.75 to 183.75 which equals 367.5 millimeters which you would then convert to inches and add to the rim size. I can never remember these conversions so I made a post a while back called Metric Versus Standard Tire Sizes in which I put some links to websites that have programs you can use to figure all this out by just plugging in the tire size- so be sure to look up that post.

Back to your question- if it came with 275/45R20 then the original profile would be 123.75 millimeters. You want to put on 255/35R20 tires which have a profile of 89.25 WHICH WOULD RESULT IN A SMALLER OVERALL DIAMETER WHICH MEANS THEY WOULD FIT. I'm not sure of the actual diameters of either of the tires because each tire manufacturer measures differently even if the numbers on the tires are the same. Most manufacturers list the actual overall diameter of their tire sizes on their website so check that out too. You didn't mention the rim width or backspacing but be careful if you get new rims in the future because they may not clear the brakes or suspension components. Also, THIS PART IS IMPORTANT: when you change the tire/rim size and the overall diameter is different from what came from the factory by 15%, (5% on cars) you must update the vehicle's software because your antilock brakes will not operate properly, the speedometer will not be accurate, and neither will the odometer. Not to mention the bone-jarring ride that low profile tires on trucks and suvs cause. Larger rims result in greater unsuspended (unsprung) weight and therefore longer stopping distances. You can also call The Tire Rack or check their website for more information on changing sizes safely within the 15% window (they label it plus 1, plus 2, or minus 1, minus 2, etc...) but remember- any change will effect the speedo, abs, etc..., it is just that 15% has been judged to be an acceptable error because it is so minor.

I hope I answered your question, I know it was a long winded answer so I put the important stuff in all caps. But check out those websites and my other post because that is a hell of a lot easier than working out the math in your head.
1998 ZJ 5.9 Limited W/242 swap
JKS disconnects & Garmin GPS
Heavy duty OME Suspension kit
with 2" coil spacers for 3.5-3.75 total lift
235/85R16 Goodyear MTR's Measure a true 32"

Life has No Value,
But Death Has Its Price.
Back to top
View user's profile Send private message

Joined: 01 Jan 2004
Location: Uzbekistan

PostPosted: Thu Dec 08, 2005 1:12 am    Post subject: Reply with quote

Good info, nixt. Moving this to the tire section. Wink
Nothing to see here.
Back to top
View user's profile Send private message
Display posts from previous:   
Post new topic   Reply to topic     Forum Index -> Suspension, Steering & Tires All times are GMT - 8 Hours
Page 1 of 1

Jump to:  
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum

Visit for write-ups, forums and clubs

Jeep®, Wrangler, Cherokee, Liberty and Grand Cherokee are copyrighted and trademarked to Chrysler LLC. is not in any way associated with or endorsed by the Chrysler LLC.
All other content is copyright Jeep Horizons 2004-2008.
Tech write-ups Links Discussion board Land use News Site Map Terms of Use

Powered by phpBB © 2001, 2005 phpBB Group