


Last Changed 5/8/2011 

Selecting tires for your RV comes after knowing the weight the tires will have to
carry. Weight capacity to carry our load RV is one issue for tire size.
The other is that if there is excess load capacity in our tires, we can have the
tire pressure lower than the Maximum PSI for the tire. This gives a softer
ride. See Tire Inflation.
The discussion will be targeted to will be targeted more to trailers than motorhome
because your have more flexibility in choosing tires with a trailer than a motorhome.

The typical tire today carries a standardized size code molded into the sidewall. The code is pretty easy to understand once you have the
decoder ring. See
Sample Tire Sizes
ST 235 / 85 R 16 E
     Load Range
    Rim Diameter
   Construction = Radial,
B = Bias
  Aspect Ratio ( the Percent value of the tread width that is the tire height)
 Tread width in millimeters
Type ST=Special Trailer, LT=Light Truck, P=Pmetric for passenger car,
For example LT235/85R16. This means you have a 16" rim, the tire tread width is 235 millimeters(9.3"), and the height of the tread from the rim is 85% of the 235 mm
or 7.85" for a total diameter of 31.7".
The issue here is for trailers. The ST(Special Trailer) tire is designed for trailers. The ST tire has a higher load capacity than a
LT (Light Truck) tire. LT tires can be used for trailers as long as the load capacities are checked to be adequate.
For autos and trucks, the Aspect Ratio has some meaning, as often a wider tread is desired. By selecting a lower Aspect Ratio (60%, 70%) the
resultant Tread Width gets wider assuming a fixed tire height. For trailers, the biggest issue of tire
Aspect Ratio is controlling the total diameter of the tire, as diameter is more critical than tread width. There will be an example
of this later.
Radial Ply is about the only tire construction you can get. The characteristics of the Radial Ply tire are so much better than Bias Ply
tires that the Bias Ply construction is usually used only for special applications.
Normally this is selected by the RV manufacturer. Sometimes, different sized rims, usually larger, can be used to get higher load ranges.
This will be covered more later.
Ply ratings do not refer to how many plies are actually used to make the tire. The ply rating system dates back to the days when cotton
was used to reinforce the rubber. Obviously, extra plies meant extra strength. The more sidewall plies a tire had, the more air
pressure the tire could hold and therefore have a higher load capacity. Today's tire construction uses about the same number of
physical plies for different Load Ranges but the cord material, rayon, polyester, nylon,
or steel, can vary in diameter giving the desired
sidewall capacity. While plies are sometimes still referred to, the more common metric is now the Load Range, a letter that denotes
the load carrying capacity of the sidewall. See
Tire Ply Ratings
These plies refer to the layers in the tire sidewall. There are addition plies, usually steel, under the tread that provide stability
for the tread but do not add to the load capacity.
.
The Load Range does not represent the same load capacity value for all tire sizes. The Load Range represents the load capacity for a given size tire.
For example, these two tires have different load capacities for the same Load Ranges
LT225/75R15 
LT235/85R16 
Load Range  Load capacity  Load Range  Load capacity 




D  2335 lbs@65 psi  D  2623 lbs@65 psi 
E  2680 lbs. @80 psi  E  3042 lbs. @80 psi 
  G  3750 lbs. @110 psi 
Another value you can find on the tire sidewall is the speed range. This should only be of concern to automobiles and trucks.
However, many RV trailer tires are rated for only 62 mph. 62 mph is probably enough for safe RV towing but not for the speeds we have
observed rigs going down the road.
When determining load capacity, the controlling factor will the element with the lowest capacity. For example a pair of G load rated tires
could have a capacity of 7500 lbs. but if they are mounted on a 7K axle, the load capacity is 7000 lbs. Beefing up the tires and axles
may not yield the results desired if the suspension is not adequate. When contemplating changes in your running gear, (tires, rims,
axles, brakes, and spring) check out all the elements.
We have observed that many RV trailer manufacturers determine the tire capacity for their trailer by subtracting the pin weight of tongue
weight from the GVWR (Gross Vehicle Weight Rating) of the trailer. Ex:

lbs. 


GVWR 
18,500 
Pin Weight 
3500 
Net 
15,000 


(4) G load rated tires 
15,000 
The manufacturer put just enough tire capacity for the GVWR of the trailer when it is on the tow vehicle. However, when the trailer
not on the tow vehicle, the weight balances changes as the load is balanced between the front jacks and the wheels.
We like to have a tire capacity large enough for the entire trailer GVWR. That leaves what the pin carries as a very
comfortable safety margin.
Actually the best number to use is after you have had each wheel of your loaded RV weighed.
Only then you can be sure that no one wheel will be overloaded.
The usual case will be that you will want tires with more load capacity after you weigh your RV with all your stuff aboard.
Or you may want larger Load Range tires so that you don't have to run at maximum pressure.
For the following examples, we will be looking at a 6 wheeled trailer. The concepts will fit motorhomes as well with the exception that
rim size options are more restrictive.
Let's start with a trailer that was delivered with LT235/85R16 D tires with a load capacity of 15,738 lbs. After weighing the
trailer with the stuff loaded, we found the trailer weight to be 17,500 lbs. We could change to LT235/85T16 E tires which would yield
a load capacity of 18,252 lbs. This is quite feasible since the 16" rims delivered with the D Load Range tires were
probably rated at 85 psi (the maximum air pressure that the rim can safely hold is stamped in the rim near the valve stem).
But let's say that after weighing the trailer, we found that the total trailer weight was 19,200 lbs. If LT235/85R16 G tires were installed,
we would have a load capacity of 22,500 lbs., more than enough. But this is not as easy as it seems. The 16" rims that the
D Load Range tires were delivered on are only good to 85 psi. To put G Load Range tires on our trailer, we will need new rims rated
for at least 110 psi to install the G tires.
What if our weighed trailer came in at 23,000 lbs. The biggest Load Range available in 16" rims is G. To go to H Load
Range tires, we would have to go to 17.5" rims. That would give us a tire load capacity of 28,836 lbs.
But wouldn't we have a problem of the wheel diameter being too large and the tires touching each other as they move fore and aft as
they rise and fall on the suspension. This is where Tread Size and Aspect Ratio come into play. The overall diameter of the
LT235/85R16 tire is 31.7". The H Load Range tire we would select for this application would be the 215/75R17.5 (there is no LT
or ST as these sizes cannot be confused with passenger car tires). This tire is skinnier, 215mm (8.5") versus 235mm (9.3")
and the Aspect Ratio is smaller so the tire height is less (215*.75= 6.35") versus (235*.85= 7.85") . The resultant diameter
for the 215/75R17.5 is only 30.5". So the 17.5" tire will easily fit in the wheel wells and not interfere with the other tires.
But there is another issue. The typical lug bolt size for a 17.5" rim is different than the typical 16" rim. The
17.5" rim usually has a 5/8" log bolt while the 16" rim uses 9/16" lug bolts. These differences also reflect a
difference of the load rating of the wheel hub. The 9/16" lug bolts are used with hubs on "7K" axles. meaning 7,000 load
capacity, 3,500 lbs. hub capacity. The 5/8" lug bolts are used with "8K" axles with hub capacities of 4,000
lbs. each.
Since we were looking for a total load capacity of 23,000 lbs., we would need to put 8K axles on our trailer. So it is not just an issue
of tires. Sometimes the rest of the suspension needs to be beefed up.
Let's say our trailer weighed in at 21,000 lbs., G Load Range tires would gives us enough load capacity but we would be running almost
at the Maximum PSI the tire is rated at. What if we wanted to be able to run our tires at less pressure than the Maximum PSI for a
little softer ride? We could use the H Load Range tires . Since we would still be within the load rating of our axles and hubs,
we would just need 17.5" rims that used 9/16" lug bolts. There are such special wheels available. We did a tire upgrade
for this very reason and it is covered in Tire Size Change. Now we could run at the same 110 psi we
used with the G Load Range tires. But now that is only 110 psi out of the 140 psi (78%) with a resultant softer ride.



