Rubicon - Wrangler Tires

Bob:

All tires have a maximum tire pressure stamped on them. That is the maximum tire pressure for that tire, not for your vehicle. That tire can be used on many vehicles, some that are MUCH heavier than a Wrangler. If you put 80 lbs in your Wrangler tires, the Jeep would ride like the tires were made out of concrete.

The recommended pressure for your vehicle is on a sticker on the drivers door, or door jam.

Tom

i will look at the door jam the next time i see this thing. it is my wife's jeep and to be honest, i feel REALLY stupid for not thinking to look at the door jam.

my original intent with my post was to find out what others were doing and maybe somebody might be using their Jeep in a manner similar to my wife. a little extra air pressure would increase the mpg a small percentage. would it be worth it? a sacrifice in ride quality with more air, yes, but would it also sacrifice safety?

i know, i know. if i was so worried about mpg, i should've bought a diesel rabbit... i did and i run about 40psi in those tires and printed on the sidewall -

35psi max

-- bob z.

"people with less brain power than you are doing more difficult things everyday"©

You shouldn't overpressure MTR's, they have a softer tread compound and you'll get excess wear in the center of the tire faster than usual.

• * * Matt Macchiarolo
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At 12/5/03 05:24, mabar bespake thusly:

Unfortunately that pressure is based on using the OEM tire, not an aftermarket one, and simply does not apply for either a specialty low pressure or high pressure tire. Particularly on a high pressure tire, the factory setting may not keep the sidewalls sufficiently straight. Plus the factory setting is often a few pounds lower than the optimal wear setting in an attempt to cushion the ride a bit. Only real way to set tire pressure is to measure the contact patch.

I run mine at 30 psi on road and 12 psi offroad. Anymore than 30 psi and it feels like you are in fred flinstones car.

Rob

I agree with this. I have a Sahara with the standard 30x9.5 Goodyear GS-A tires still on it.

I used my 6yo daughter's sidewalk chalk. Came up with 30psi as being "right" with perhaps a bit more necessary in the front. The sticker says

29psi, the tires say 44psi max. Use the chalk to draw a thick line across your tires and drive STRAIGHT ahead for 50-100 feet. See if the chalk has worn off evenly. Adjust air as necesary and repeat until you get it right.

Or buy a $500 pyrometer and measure the tread surface temperature while driving down the highway at 60mph until the temp is even from side to side.

My Jeep was delivered to me with the tires at 45psi. Rode like shit and hunted all over the highway with that much pressure. I asked the dealer and they told me to use the pressure on the tire (!) not the sticker. Thankfully I'm not that stupid.

You can estimate the minimum pressure 'allowable' on the road by looking up the tire manufacturer's max load (at max PSI). You need to assume a

50/50 weight split front/rear, which for a wrangler is accurate if there isn't a winch on the front, otherwise you need to modify this procedure to take the higher of the front/rear loads. Both should always be the same pressure on a 4x4 even if it isn't necesary for load reasons.

Take the max PSI, multiply that by 1/4 of your vehicle weight, and divide by the max load for the tire. That is close to the minimum pressure that is OK for the road. For *my* TJ Sahara that is about 26 PSI with my wife, daughter, and a tank of gas (truck stop scale!)

You can go lower > Unfortunately that pressure is based on using the OEM tire, not

On 05 Dec 2003 08:40 AM, Lon Stowell posted the following:

The Goodyear Wrangler MT/R _IS_ the OEM tire on the Wrangler Rubicon, so the pressure listed on the door jamb label is correct.

In any case, the MT/R is not a "specialty high or low pressure tire." It is certainly a specialty tire, but for running on the road you should use the same pressure as you would any other tire of the same size and construction on that vehicle. Off-road, you will probably want to air down for better performance but that isn't really relevant here.

I'm running MT/Rs (aftermarket) on my TJ and I use the door jamb pressure, but I'm running 30" tires which are a size available from the factory. Bigger tires will most likely need to run at a slightly lower pressure, but that doesn't make them a low pressuer tire.

---------------------------------------------------- Del Rawlins- del@_kills_spammers_rawlinsbrothers.org Remove _kills_spammers_ to reply via email. Unofficial Bearhawk FAQ website:

Bob:

My '02 Wrangler came with 30 x 9.5 x 15 tires. The factor door sticker says

29 lbs.for all 4 tires. I changed to 31 x 10.5 x 15 and now run with 28 lbs. Seems to be just about right.

Tom

On 05 Dec 2003 10:10 AM, Tim Hayes posted the following:

It sounds as though you managed to verify that the Jeep factory got their recommendations for stock tire pressure right. The difference is certainly within the margin of error of your gauge.

Actually there is nothing special about a 4x4 that requires the front tires to be the same as the rear. Matter of fact, I used to drive a pickup truck for work that needed 55psi in the front tires, and 80 in the rear. Any more than 55 in the front would have led to uneven treadwear, while any less than 80 in the rear and it wouldn't have handled the weight.

10+ years in the tire business and I have never seen or heard of that method being recommended, though it seems to make some sense. If you have any documentation supporting it I would be curious to read it.

---------------------------------------------------- Del Rawlins- del@_kills_spammers_rawlinsbrothers.org Remove _kills_spammers_ to reply via email. Unofficial Bearhawk FAQ website:

The chalk method works great for oversized tires.

I used it on my 31x10.5's and 26 psi front and rear worked. They wore out totally even for tread wear.

Same for my new 33x9.5's, at 35 psi like they came from the tire shop, I only have 4" of tread touching. Drop them to 26 psi and I am only missing 1/4" on the edges. This turns out to be too low for the tall sidewall on the 33's and the edges scuff, so I use 28 psi.

A Jeep CJ or Wrangler has almost a 50/50 weight split on the tires when not loaded. I believe the Cherokee is close to the same.

They both call for even front to back air pressure.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's

Del Rawl>

I run mine at 27psi. n.

When I had 31x10.5 BFG muds on mine, I ran at 26 in town and 28 highway when loaded.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's

On 06 Dec 2003 10:33 AM, me posted the following:

That is way more pressure than you need and it will most likely lead to abnormal wear.

---------------------------------------------------- Del Rawlins- del@_kills_spammers_rawlinsbrothers.org Remove _kills_spammers_ to reply via email. Unofficial Bearhawk FAQ website:

That is actually pretty dangerous.....

At that pressure you only have about 1/4 of your tread actually touching the road when you drive straight.

This can lead to a fast end to end swap if you hit water or snow. They will hydroplane fast and quick and you will be doing 360's quicker than you can blink.

The water will get under the other 3/4 of the tread and literally lift you right off the road.

I tested my 33x9.5's with 35 psi in them and only had 4" of tread on the ground. I got the best grip at 28 psi.

When I used 31x10.5's they wore out perfectly even all across and I was running at 26 psi city, 28 highway.

Now If I had those tires on a 1 ton truck and carried a load, I would run them close to the top rated pressure.

But a Wrangler is only a 1/4 ton truck.....

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's

Mike:

I agree, 48 lbs. is WAY TOO MUCH pressure for that application, but as far as hydroplaning goes...

A tire with lower than recommended pressure will hydroplane easier, quicker, and at lower speeds.

A tire with higher than recommended pressure will hydroplane less, and usually at higher speeds.

Tom

I drove my brother in laws Volvo once with 60 series tires on it and that thing couldn't go over 50 mph in a heavy rain without losing the steering control due to hydroplaning.

Turns out the tires were at the sidewall max pressure, not the 32 psi recommended. The water had free space to get under the sides of the tread lifting the tire up off the road.

I do agree too low is bad too.

Mike

mabar wrote:

Mike, the higher the tire pressure, the faster a tire can be driven without hydroplaning. That's because the narrower the footprint, the less tire surface the water can push against. That's why snow skiis wouldn't work for waterskiing unless you were going 90 mph. ;)

A simple formula taught to pilots to determine the estimated speed at which a tire will hydroplane is 9 times the square root of the tire pressure equals the estimated speed a tire will hydroplane at. So for example if a tire were inflated to 25 psi, it would likely hydroplane somewhere near 45 mph. If that same tire were inflated higher to 36 psi, it'd likely hydroplane at 54 mph. So the air pressure the tire is filled to is directly proportional to the estimated speed at which it will hydroplane at. :)

Jerry