Tires: tire choice, ford f 350, goodyear wrangler

Question
QUESTION: I have a F450 which I changed to a super single rear end setup.  Meaning I am having wheels custom made to fit correctly so I only have one wheel out back.  The plan is to use 20x8 wheels and an
LT265/75R20 Tire.  Load range E. For everything I do this would be more than adequate.  However, I have looked at a camper and noticed it weights approx. 4k. wet. The truck suspension is fine but I thinkt the load range on the tires will be to low.  I could go to a G or H rating or even a bit wider, but of course those start getting pricing also.  I would apppreciate your thoughts.

thanks,

Jack

ANSWER: Jack,

What concerns me most is the loss of load carrying capacity.  In this case, it's a "devil's in the details" kind of thing.

Unfortunately, I can not find a listing ANYWHERE for an LT265/75R20.  I've done a google search and while I can see some postings that you made, I can not find any load / inflation tables that include that size.  In fact, the only place these things seem to be mentioned is in discussion threads - notorious for being inaccurate.

So if you have the tires now, what brand are they?

The second thing is about the original load carrying capacities of the tires.  I think the original tire size was 225/70R19.5, but I do not know the original inflation pressure.  Is the vehicle tire placard still affixed to the doorpost?  What does it say?

Do you know what the original payload was for the vehicle (should be in the owners manual)?

---------- FOLLOW-UP ----------

QUESTION: The OEM tires are G rated with a pressure of 110 max.  Payload max is 14,500.  the thing is I don't need to haul anywhere near this number.  The tires I was considering (before the camper idea) was the OEM tire for Ford F-350 Harley D. Truck.  That tire is a Goodyear Wrangler 275/65/20  E.
This tire has an inflation max of 3,750 therefore 7,500 total.  My thought was maybe there is another 20"tire with a larger rating because of width or G rating. I have just not been able to find any.  Any thoughts.  Also, based on the above tire choice, what would be the heaviest camper you would use?  Figuring five weekend trips per year all hwy.  Thanks for the help.

Jack

ANSWER: Jack,

I think you are confusing GVW with payload.  Payload is the amount of load the vehicle can handle before it maxes out on the GVW.  This is important, because the camper you are considering is payload.  

If, for example, the rated payload is 6,000#, then a 4,000# camper reduces the actual weight the rear tires need to carry by 1,000# each (and less if we assume the weight is partially carried by the front tires).

Remember I said the "devil is in the details"?  Well, it turns out that the F-350 Harley Davidson comes with a LT275/65R20 Load Range E (which is a different size than discussed earlier), and the load carrying capacity at 80 psi is 3750#.

By contrast the load carrying capacity of a 225/70R19 LR G at 110 psi is also 3750# for dualled applications.

So if I calculate the loss in load carrying capacity, it is 7500# - and that does not make sense to me as that is preciously close to what I have seen as the published payloads for F-450's.

So we need to check on some more details:

1)  Does the vehicle tire placard actually call for the rear tires to be 225/70R19.5 inflated to 110 psi?

2)  What are Ford's published payload for the particular version you have?


---------- FOLLOW-UP ----------

QUESTION: The door placard states 225/70R 19.5F is correct and 110lbs is max pressure.  The payload for this truck is 9,000lbs. Front axle is rated at 7,000 and the rear is rated at 12,000.  What are your thoughts?

Answer
Jack,

It isn't clear that the vehicle tire placard says that the proper tire pressure is supposed to be 110 psi.

But assuming that is so, a 225/70R19.5 has a load carrying capacity of 3970# at 110 psi for a single application and 3750# for a dualled application.  That means the front tires can carry up to 7940# - or they have a 940# (13%) reserve capacity.  That is good.

Again assuming the vehicle tire placard says the tire are supposed to use 110 psi, then the tires on the rear have a total load carrying capacity of 15,000# - or they have a 3000# (25%) reserve capacity.  That is also good.

It is now important that we not gloss over the issue about what the vehicle tire placard says for inflation pressure.  Because load carrying capacity of tires is directly linked to the pressure, and the GAWR's you quoted are based on that being as stated.

I have heard that F-450's have a payload capacity of 6,000#.  If this is true, - and you need to check on this - and you plan on putting a 4,000# camper on the truck, then that is an unused capacity of 2,000#.

That means the actual rear axle load will be in the vicinity of 10,000# - assuming that 100% of the payload is carried by the rear tires and none is carried by the fronts.  This isn't a bad estimate - and certainly is a conservative way to estimate the load.

However, the proper way to do this is to know for sure what the payload capacity is for the truck and adjust the actual loads based on that information.  What you should aim for is at least the same percentage reserve tire capacity, and the same actual reserve capacity in pounds would be even better.  That way, you can be sure you are matching what the Ford engineers designed into the truck.

But returning to the situation at hand:  If you are planning on using LT275/65R20 Load Range E tires inflated to 80 psi - the maximum - the rear tires have a combined load carrying capacity of 7500# - a 2500# overloaded (33% overloaded).  

This is a recipe for disaster!   You need to rethink this whole thing.  

Perhaps some of my assumptions are incorrect, but even if they are off a little, the figures above do not indicate you can even put a load in the truck without altering the load carrying capacity strategy the Ford engineers employed.