Tires: rv tires, edmonton alberta canada, rv tires

Question
QUESTION: Hello Barry Smith,

My name is Shawn Smith from Edmonton, Alberta, Canada.

My wife and I just bought a travel trailer for camping.  Standard tires for the trailer are: ST225/75R15 - 4 tires total dual axel.
They are also a load rated "D" tires.  The "Shipping weight" or "Dry weight" is 7653 lbs and the Carrying capacity is 3347 lbs, and the tongue weight is 840 lbs (Hitch weight).

My Question is:
In your expert opinion, do you feel that the trailer company is providing the bare minimum tires for the loads that this trailer can carry?  And if so, do you feel that it would be safer to upgrade to a load rated "E" tire?  Also what would a safe speed to travel when towing this travel.  Our highways limits are 110 Kph (68 MPH)

I thank you in advance for your advice,

Shawn Smith

ANSWER: Shawn,

Let's see:

An ST225/75R15 Load Range D has a maximum load carrying capacity iof 2540# at 65 psi. 4 X 2540 = 10,160#.

If you combined the trailer's shipping shipping weight with the capacity you ought to get the Gross Vehicle Weight (GVW).  So 7653 + 3347 = 11,000#.  

Then subtracting the tongue weight to get the load on the tires: 11,000 - 840 = 10,160#. The same of the tire load carrying capacity.

Mmmmmmm.  I am always suspicious when things like this come out so even.  So let me start picking it apart.

I have heard that you need between 10% and 15% of the weight of the trailer on the tongue.  So the tongue weight should be between 1,016 #and 1,524.  That is bigger, and would reduce the load on the tires.

Using the 15% value, the load on the tires drops to 8,636 or 85% of the tire load capacity.

Since this is the largest tongue weight, this would be a maximum.

Over the years, I am of the opinion that you need a 15% unused reserve in trailer tires, so I would say, this trailer came with tires that are at the limits, no matter how I do the calculations.

It would be safer to use a Load Range E with the higher pressure to get the higher load carrying capacity.




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

QUESTION: Thanks Barry.

In regards to the travelling speed with the trailer - Would the load E rated tire allow me to tow at our posted 110 km's/h (68 miles/h) without damaging the tires or increasing the heat stress?

Answer
Shawn,

ST tires are speed restricted to 65 mph.  In order to gain a 70 mph speed, the pressures have to be increased 10 psi and that is applicable regardless of the maximum pressure written on the sidewall (and that applies to both Load Range D and Load Range E).  In order to gain a 75 mph speed, the load has to be reduced 10% in addition to the pressure increase.

So using a Load Range E would make a great deal of sense, particularly in you start off with more inflation pressure.