Tires: Pressure for Kumho Tire on EuroVan Camper, VW Eurovan, VW camper

Question
I would like to know the recommended tire pressures for my 2001 VW EuroVan Camper (Winnebago Conversion)with upsized tires.

Vehicle Sticker: GAWR 3329 lb Front, 3108 lb Rear, 225/60R16 tires at 49 psi.
OEM Tire: Dunlop SP Sport 2000 E, 225/60R16 102H Reinforced, Max Pressure 49 psi, Max Load 1874 lb
New Tire: Kumho Road Venture APT KL51 235/60 R16 104H Extra Load, Max Pressure 51 psi, Max Load 1984 lb

Answer
William,

Are you sure the tire placard says 225/60R16 102H at 49?  My book (Tire Guides) says the placard size is the same but the placard pressure is 38 psi front / 29 psi rear.  This makes more sense than the value you quoted.

Assuming my book is correct, then a 235/60R16 104H can carry the same loads at 36 psi front and 27 psi rear.

William wrote at 2009-09-04 05:52:46
Yes, I am sure the tire placard says 225/60R16 102H, Cold Inflation Pressure 49 PSI, Front and Rear. The numbers provided by Barry Smith are for a lightly loaded VW EuroVan, including the German made Westfalia Weekender. My question regards the Camper model, with 900 lbs of additonal Winnebago Conversion items, including a pop top with structural reinforcements around the roof line. Winnebago recommends maximum pressure of 49 PSI to achieve a 102 Load Rating with the OEM tire on this tall, 6000 lb vehicle.

Many technical references used by mechanics, tire installers, etc., list the VW specifications for this vehicle, but in fact the Winnebago placard should superceed the VW cargo van specs. EuroVan Camper Owners have experienced tire failures due to tire pressures maintained at the 40 PSI range. (Ref: EV_Update on YahooGroups - http://autos.groups.yahoo.com/group/ev_update/message/58199 up to message 58226, then message 58789)

Please update the response to my original question using 49 PSI Front and Rear as the recommended pressure for the OEM tire.

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William,

Thanks for the update.  I got your response through my email.  As you saw, I am taking some time off - to care for my elderly father while he recovers from heart surgery.  Rather than disappoint folks with questions that get unanswered for days, I put myself on vacation.  Unfortunately, your followup got caught in the timing.

As you saw, the confusion about what the vehicle placard says leads down a certain path.  

RV conversions have always been problematic.  Most converters will work within the design limits of the truck - and that means the original tire placard still applies.  This is good because the placard information of the original vehicle is catalogued by a company called Bennett-Garfield, who publishes a book called "Tire Guides" with this information in it.  I always go to this book to see if everything matches up - and if it doesn't, I always note that in my responses so a followup can be made if the placard does indeed say something different.

But sometimes RV converters don't stay within the design limits of the original vehicle  (BTW, Stretch Limousines are notorious for this!) and that means the original tire placard is NOT applicable and the responsibility for a new placard falls to the RV converter.  For some reason, RV converters do not ask for assistance from tire manufacturers - and when they do, they don't always follow the tire manufacturer's recommendations.  You will see how that plays out later in my posting.

One of the fallouts of this is that the placard information for RV's is not catalogued anywhere - and more importantly, there isn't anything except the tire placard on the vehicle to alert that there is a potential situation.  This means that if someone were to go to Tire Rack and put in the vehicle information, they sometimes get wrong information.  

But there was something in your original post that set off alarm bells for me - it was the 49 psi.

A 226/60R16 102H is an Extra Load tire - and those types of tires max out - load vs inflation pressurewise - at 41 psi.   It is extremely rare for a vehicle manufacturer to specify more pressure than that.  BTW, for Standard Load tires, the max load occurs at 35 psi(English units) or 36 psi (metric units) and the same logic applies.

I am of the opinion that Winnebago has misinterpreted the tire standards and specified an unrealistic pressure.  This is not the first time they have done so.  And there are 2 clues here.

1)  The fact that Winnebago would add 900# to the vehicle and not change tire size (which also means a larger load carrying capacity)

2)  That folks are experiencing tire failures in the 40 psi range.  This pressure ought to be adequate for load carrying capacity purposes if the tire size and load index is specified correctly.

And I will add one more:  Passenger car tires used light truck, trailer, and multi-purpose vehicles need to be derated 10% load wise.  This vehicle would qualify under the "multi-purpose" provision - as would most vans.

So the load carrying capacity of a 226/60R16 102H with a max pressure of 49 psi and a max load of 1874# in reality has a load carrying capacity in this type of service of 1687# at 41 psi.  So if the GAWR (Gross Axle Weight Rating) - the weight that axle system (which includes tires and wheels!) is designed to carry - is 3329#, then the tire needs to be able to carry at least half that - 1665#.  Ergo, the tire is being used at its maximum.

One of the lessons from the Ford / Firestone situation a few years back, was that the load carrying capacity of a tire needs to be larger than the minimum in order to assure reliable operation.  I had a professor in college who used to say it this way:  "Overdesign / Underutilize" - and this lesson goes to the "underutilize" part of that saying.

I am of the opinion that tires need to have a minimum of a 10% unused load carrying capacity - and 15% ought to be the design standard. So if the GAWR's are indeed correct - and I have my doubts based on Winnebago's past performance - the tire specified for this vehicle ought to be a minimum of a 105 Load Index, and more like a 107 Load Index

I wonder if Winnebago meant to specify a 225/60R16C 105/103H.  Then the 49 psi pressure specification would make perfect sense.  That tire would be adequate for this vehicle (An additional complication when specifying tires is that "C" type tires don't have to be derated 10% as they are already designed for this type of service.)

I looked up to see if such a tire exists and apparently Continental is the only one that makes one - but that also makes sense since both VW and Continental are based in Germany.  

Now back to your original question:  

I am of the opinion that the 235/60R16 104H tires are still inadequate for the vehicle.  And any pressure recommendation I make has to be based on sound engineering judgement.

If you feel you have to use the 235/60R16 104H I would recommend you stay at 49 psi.  As you've read above, I have reservations about this.

I looked up on Continental's web site, and I see that the 225/60R16C 105/103 isn't listed as available in the US.  But they do list a 215/65R16C 109/107 - and that seems to to the solution to this problem.  it is slightly larger in diameter (10mm) and has the requisite load carrying capacity.  Unfortunately, it has an R speed rating - and that means care has to be taken to keep the speed of the vehicle down - especially in hot weather.