Various other FiberglassRV members and information from several tire company and tire retailer web sites have identified the one factor which I would be careful to consider in using a passenger-service tire in trailer service: the "P" type tire capacity must be reduced by 10% to get the capacity in trailer service.
In the original sizes used by most trailer manufacturers the tire are very heavily loaded by modern automotive standards. Since the capacity of a tire is fundamentally limited by size and pressure, they often must be run at more than the typical 35 PSI of automotive tires to have sufficient capacity - that makes it hard to find a suitable passenger-type tire in many cases. Basically, if the trailer tire has adequate capacity in load range B, then it only needs 35 PSI and passenger tire might work; if a higher load range is needed, then 50 PSI (load range C) or 65 PSI (load range D) is called for.
Although I know I'll get some flack for saying this, I can't believe that there is any meaningful difference between a specific trailer tire ([b]Special Trailer, designations starting with "ST") and any commercial or light
truck tire (Light Truck designations start with LT). I think the only reason ST tires exist is that there are no LT tires in the small sizes used by small utility and travel trailers. It cannot be coincidence that once you get larger than the largest ST tires (e.g. ST235/80R16 in a Goodyear Marathon
), your're into LT size territory. All of the largest commerical and RV trailers use LT tires - or commercial vehicle tires. When an LT tire is used on a trailer, its capacity does not need to be rated lower (as a P tire does).
In the end, to replace an ST185/80R13 tire (about 24" diameter) there will be no LT tire available (too small for LT), all ST tires will be 75-series or 80-series (so on a 14" wheel they will be too tall), and you are left with P tires if you want to match diameter
Capacity in passenger car tires, and now in LT tires, is given as a load index
. If you need to support (for example) 1600 lb on the trailer axle
, that's 800 lb per tire (before accounting for that 10%) which happens to match a load index of "77". I used Goodyear's search tool for 195/70-14
, and found that in this size P-type tires are typically load index 90, which looks like lots to me. A tire with a higher speed rating (that's the letter after the load index) will likely have stiffer sidewalls for better control.
For comparison, the T@B trailer is equipped from the factory with low-profile Fulda passenger car tires. If it's good enough for the Europeans, and for the Thor division selling these trailers in North America, then perhaps the magic ST designation is not required.
This subject came up at the end of the recent topic [b]Blue boler pix
. Lance is also using 14" passenger car tires, but hasn't shared with us yet the specific size or type. I'll repeat here my opinion regarding radials versus bias-ply from that thread:
There's certainly nothing wrong with radial tires, for just about any application. The most common recommendation I have seen in this forum is for the Goodyear Marathon, which is available only as a radial.
By the way, Goodyear overhauled their website within the last week or so, and old links will probably no longer work).
[b]Correction and edit: Andy spotted that I incorrectly omitted a reference to the very common use of commerical vehicle tires (not just the LT type) on larger trailers - I corrected this with an addition to the text above.