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Old 11-20-2018, 11:45 AM   #1
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Tow vehicle tire pressure

From the ETI Owners Manual. Page 10.

"• Inflate the rear tires of the tow vehicle to their maximum cold pressure. (See the maximum pressure rating on the tire sidewalls.)"

On my tow vehicle the max carrying weight per tire is 2,337# at a max inflation pressure of 51#.

The recommended tire inflation pressure on the door post is 32#. Nowhere in the Jeep Owners Manual does it talk about increasing the rear tire pressure to the max when towing.

Wondering what others are doing in regards to increasing the tire pressure when towing.

Thanks

Ed
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Old 11-20-2018, 11:54 AM   #2
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The maximum load rating of tires is always stated at the maximum cold inflation pressure. At lower pressures the max load rating is less. There are so many different vehicle / trailer / tire possibilities that it's hard to generalize a "one size fits all" recommendation, you have to consider the load that is on your rear tires with the trailer tongue weight added and then consider what pressure may be needed to safely handle that. Many tires' maximum pressure is well above the actual weight they will be carrying - I run LT rated tires on my pickup whose max pressure is 80psi, but they don't need nearly that much to safely handle the load, they are just very heavy duty tires and if inflated to the max pressure would ride very rough. You can also experiment with different inflation pressures and see how it affects your ride.
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Old 11-20-2018, 12:12 PM   #3
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I recall the manual stating to inflate the tires to the max on the sidewall which is 50 psi, for the trailer, but do not recall anything about the tow vehicle which you should follow the yellow door sticker amount? Anyone else?
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Old 11-20-2018, 12:46 PM   #4
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I just read the on line manual quickly now and everything refers to the trailer tires.....
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Old 11-20-2018, 01:00 PM   #5
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Quote:
Originally Posted by cpaharley2008 View Post
I just read the on line manual quickly now and everything refers to the trailer tires.....
The quote in my original post came from Page10 of the Escape Owners Manual. Chapter 3; towing and leveling.

Here it is again.

g.
“• Inflate the rear tires of the tow vehicle to their maximum cold pressure. (See the maximum pressure rating on the tire sidewalls.)“

Perhaps I not reading this item correctly?

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Old 11-20-2018, 03:04 PM   #6
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Ed, that is what the Escape Owner's Manual says, and you are reading it correctly.

I don't take advice regarding the operation of my motor vehicles from the manufacturers of other vehicles. It may make sense in some cases to increase the tug's rear tire pressure when towing, some vehicle's owners manuals say to do that, and I have increased the rear tire pressure moderately in some cases, but a blanket instruction to use the maximum inflation pressure regardless of the tow vehicle should be disregarded.

A reason for the higher pressure could be to increase tire load capacity, but the vehicle manufacturer's recommended inflation pressure is high enough for the full rated axle capacity, and more air in the tires won't increase the capacity of any other component of the system (bearings, suspension, structure) and so does not permit exceeding the rated axle capacity.

Another reason for a higher pressure could be to increase the tire's lateral stability - and that's why I have increased it - but too much pressure can reduce traction and cause handling and stability problems. This is not something to play with blindly.
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Old 11-20-2018, 03:23 PM   #7
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Thank you, Brian.

I also found the statement odd but then, I’m a newbie with Escape so thought I would ask the Pros.

I’ve owned both an Arctic Fox and Casita. Neither recommended increasing the tire pressure.

Neither does my Jeep Owners Manual.

I’ll stick with the pressure recommended on the column in the Jeep.
33#

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Old 11-20-2018, 04:53 PM   #8
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Ed,
Do you have any questions about your new 21 and also check your Jeep spare tire, I found out that my truck did not have a full size spare acceptable for towing.
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Old 11-20-2018, 06:07 PM   #9
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Quote:
Originally Posted by cpaharley2008 View Post
... also check your Jeep spare tire, I found out that my truck did not have a full size spare acceptable for towing.
Ed has a 2018 Jeep Grand Cherokee Trailhawk. It comes with 265/60R18 tires on 8" wide alloy wheels. Based on the load capacity, these are Standard Load - load index 110 - tires, and not P-type. Apparently both a compact spare and full-size spare are unavailable, but a steel spare wheel is standard, which means that it has a spare which is at least approximately the full diameter (so not "compact"), but not the same as the 265/60R18 regular tires (so not "full size"). The placard on the door frame which gives the inflation pressures will show the spare tire size and type.

The Jeep Owner's Manual says not to exceed 50 mph (when towing, or when not towing) with the spare... but you can still use that spare while towing.

The manual says to put the flat regular tire in the cargo area (not specifically the spare tire well), so a regular 265/60R18 tire might not fit in the spare location. If it does fit, you could get another matching tire and have no restriction on towing (or even just driving when not towing) after getting a flat, but I don't know if this is a big enough deal to bother.

Online discussions suggest that this vehicle's spare tire well is just big enough to hold a 245/65R17; the 265/60R18 is about 20 millimetres wider and an inch or 25 mm larger in diameter, so that 18" tire likely wouldn't fit in the well. The same discussions suggest that the spare might be a 245/60R18, which is close enough to the regular size to work, but mismatched enough (in both width and diameter) that Jeep wants you to keep the speed down.
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Old 11-20-2018, 06:24 PM   #10
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There is always the roof rack, that is what Jon V did with his full sized spare in his Rav4 and the E17. It amazes me when auto manufacturers fail to think far enough ahead when designing options what impact these options may have on the ability to use the said vehicle with such options installed. The oversized tires on both my Ram and other vehicles is an example of such.
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Old 11-20-2018, 07:05 PM   #11
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This is weird. Never would I have thought that the spare was not the same size as the main tires. Brian knows more about my TV than I.

Brian said
"...spare might be a 245/60R18, which is close enough to the regular size to work, but mismatched enough (in both width and diameter) that Jeep wants you to keep the speed down."

I looked at the spare just now. It's a completely different brand, Kumho vs Goodyear Wrangler All Terrain, Kevlar reinforced and a different size P245/65R18 110H M+S vs 265/60R18 M+S.

I just e-mailed the Jeep dealer with this information and asked why I wasn't told this during the purchase. Not that its going to get me anywhere, but it made me feel better.

And all these contortions to save a buck.

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Old 11-20-2018, 08:31 PM   #12
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Quote:
Originally Posted by EdColorado View Post
Brian said
"...spare might be a 245/60R18, which is close enough to the regular size to work, but mismatched enough (in both width and diameter) that Jeep wants you to keep the speed down."

I looked at the spare just now. It's a completely different brand, Kumho vs Goodyear Wrangler All Terrain, Kevlar reinforced and a different size P245/65R18 110H M+S vs 265/60R18 M+S.
They supplied a size of spare which matches the diameter of the regular tires very closely, so there's no concern with the diameter as far as driving (including the behaviour of the AWD system). The spare tire well must be large enough to allow this closer match (than what has been used in previous years and other Grand Cherokee variants), and is likely large enough in diameter to fit the regular tires.

The width difference is as expected. It's possible that the regular tires won't fit in the spare tire well, or might fit but not with the tools tray. It would be reasonably easy to check, for anyone willing to jack up the vehicle and pull one of the regular tires off.

The narrower tire is an Extra Load (XL) type, possibly so that it can have the same load capacity as the wider standard load tires (it does have the same load index); that alone would lead them to using a different model of tire. Since it doesn't match the rest of the tires in size there's no need to match in model, even if it were the same brand.

The spare is a "P" (passenger car style) tire, not a "T" (temporary use) spare-only tire. It's a perfectly good tire, just not as wide; it actually has a substantially higher speed rating than the regular tires, and is more capable in some ways than the regular tires of lower trim levels of the same vehicle. The manual's direction to keep the speed under 50 mph is apparently due only to the width mismatch, not the tire type or construction or load capacity. This is the same situation as with the Ford F-150 - the dissimilar spare, not temporary type - as discussed in an earlier thread.

regular tires: Goodyear Wrangler All-Terrain Adventure with Kevlar® 265/60R18 110T
possible model of spare: Kumho Solus KL21 P245/65R18 XL 110H

Quote:
Originally Posted by EdColorado View Post
This is weird. Never would I have thought that the spare was not the same size as the main tires.
...
I just e-mailed the Jeep dealer with this information and asked why I wasn't told this during the purchase. Not that its going to get me anywhere, but it made me feel better.

And all these contortions to save a buck.
Although I'm sure that most buyers are not aware of the difference, and most owners are probably not even aware of what is in the spare tire well of the vehicle that they are driving, Jeep doesn't claim that the spare is the same. There are lots of things they could tell you, but there isn't really a reason to mention this or anything else that isn't either a selling feature, or something which is legally required, or something the buyer is asking about.

It probably saves a few bucks, but it also saves weight (maybe only a couple of pounds), which is very valuable to the auto designer. The 265 mm wide tire might not fit in the spare tire well, which would be a more critical difference if that's the case.
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Old 11-20-2018, 08:47 PM   #13
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Brian

The Kumho tire you referenced is the spare tire I have.
Just took another look.

Thank you again.

I’ll chalk this up to a learning experience and put the issue to rest.

As far as putting a full size spare on the roof rack, the days when I could climb up a ladder balancing a tire on my head are long past. Today’s plan is to call road service and have some young buck change the tire. :-)


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Old 11-20-2018, 08:49 PM   #14
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Now, back to the original topic of blindly pumping up the tug's rear tires...

Quote:
Originally Posted by Brian B-P View Post
A reason for the higher pressure could be to increase tire load capacity, but the vehicle manufacturer's recommended inflation pressure is high enough for the full rated axle capacity, and more air in the tires won't increase the capacity of any other component of the system (bearings, suspension, structure) and so does not permit exceeding the rated axle capacity.

Another reason for a higher pressure could be to increase the tire's lateral stability - and that's why I have increased it - but too much pressure can reduce traction and cause handling and stability problems. This is not something to play with blindly.
So, how would this logic work with the tires on this Grand Cherokee?

Quote:
Originally Posted by Brian B-P View Post
possible model of spare: Kumho Solus KL21 P245/65R18 XL 110H
Tires built to Tire & Rim Association "P" standards typically have a 51 PSI maximum inflation pressure, but as shown in load-inflation tables they only need 42 PSI to reach their maximum capacity. They can be inflated to 51 PSI, but that is allowed in the design of the standard only to make lower rolling resistance possible, and doesn't help load capacity at all.

Quote:
Originally Posted by Brian B-P View Post
These are not "P" tires. The corresponding load-inflation table (which is from the European Tyre and Rim Technical Organization) shows that only 36 PSI would be required for maximum capacity (in any standard-load tire following this standard), and pumping them up to 51 PSI - while allowed and safe for the tire - would be even more pointless if trying to add load capacity.

In either case, maximum pressure could mean more sidewall stiffness, but almost certainly less traction so the vehicle's stability may be worse.
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Old 11-20-2018, 09:00 PM   #15
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Quote:
Originally Posted by EdColorado View Post
As far as putting a full size spare on the roof rack, the days when I could climb up a ladder balancing a tire on my head are long past.


I have seen some slick rack designs which would help with this, but for ordinary vehicles on normal roads there's no point.

Quote:
Originally Posted by EdColorado View Post
Today’s plan is to call road service and have some young buck change the tire. :-)
I think that's the best plan for many people in most cases. For large trucks, buses, and RVs that's the only option, as they don't even carry a spare, and wouldn't have the tools required to do the change even if they had one. While an SUV's tires are much more manageable and the wheel nuts are not nearly as tight, it's still a bit much for many people to handle.
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Old 11-20-2018, 09:15 PM   #16
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My KO2s will go to 80 psi cold But I run 45psi towing and 32 psi empty.
Taking ride and tire wear into account .
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Old 11-20-2018, 09:32 PM   #17
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I have always followed vehicle manufacture for my tow vehicle. For trailer I use the tire manufactures guide related to PSI and weight rating for the tire. I also take in account air pressure increasing as tire heats up. Example if I my trailer tire max pressure is 65 psi I will run a lower pressure such as 57 psi which is more then adequate for the load I'm carrying. My tire pressure may raise up to 65 psi depending on outside temps and elevation.
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Old 11-21-2018, 04:55 AM   #18
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specified inflation temperatures are always 'cold', and allow for thermal expansion. a hot tire on the highway can be up over 200F, so the difference between cold=40F and cold=80F isn't actually that big of a deal.

Always go by the VEHICLE's tire pressure ratings, NOT the tire's MAX pressure on the sidewall.... the vehicle's pressure ratings are based on the weight of the vehicle, while the tire pressure ratings are based on the max load for that tire, which quite likely is considerably higher than the vehicle. The tires on my Tacoma say 80 PSI, but the Tacoma is happy with like 36 PSI. My F250 requires 70 PSI in the rear at max load, but I run about 45 PSI when its mostly empty.
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Old 11-21-2018, 07:16 AM   #19
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The only Jeep with all 5 tires matching are the Wrangler model tires/wheels since the spare is on the back outside.
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Old 11-21-2018, 10:13 AM   #20
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Quote:
Originally Posted by cpaharley2008 View Post
The only Jeep with all 5 tires matching are the Wrangler model tires/wheels since the spare is on the back outside.
You are correct. The Wrangler is probably the only Jeep model where Jeep can pass off a different tire without the customer being aware.

As part of my education, I reread the Grand Cherokee Owners Manual. There are no less than FIVE spare tire possibilities on this model.

Spare tire matching original equipment
Compact spare tire
Collapsible spare tire
Full size spare for temporary use (I think this is what I have)
Limited use spare (or maybe this. I dont know)

Whatever.
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