Robin wrote:
I want to replace the tires on my Komet trailer. The original tire
size is P165/80 R13, which is now hard to find.

The choice seems to be either a Firestone passenger car radial in the
correct size or a slightly taller Towmaster made by Greenball. The
trailer tire will have a taller but stiffer sidewall. It will come a
little closer to the fender, but my mechanic says less than half an
inch closer. The size is 175/80 R13.

I plan to tow about 75 mph most of the time. I really want to avoid
sway.
Don't use trailer tires (ST type) if you tow that fast. From an article
I wrote for the ASH 26 E "tech site":

Commonly available tire types are P (passenger car), LT (light truck)
and ST (special trailer). While each tire series shares basic
construction methods, the details vary meaningfully among the three.

There are other differences:

1. P tires use a load rating system that is different from ST and
LT tires; generally, you have to discount their rating 10% to
get the equivalent ST or LT rating.
2. ST tires speed rating is 65 mph. The speed rating increases to
75 mph IF you increase the tire pressure to 10 psi above the
pressure required for your load at 65 mph. These are not the
tires for folks that like to drive 80 mph across the Nevada
highways in 100 deg F temperatures.
3. ST tires come in both bias-belted and radial construction. The
only advantage to a bias-belted trailer tire is it's cheaper.
4. LT tires are a bit "stiffer" than either P or ST tires, and are
available in speed ratings to at least 118 mph.

P tires might work fine, but I don't know what speed and load ratings
would ensure this. If I decided to use P tires, I'd use them at the
pressure that gave a load rating of ~40% more load rating than the load
they has to carry. I'd choose a speed rating at least 20 mph higher than
the speed rating on my tow vehicles tires. Rules of thumb - no real
documented tests.

Currently, I'm using Michelin LT tires on my Cobra, 105 mph rating.

If avoiding sway is really important to you, you should drive slower.
It's very effective.

--
Eric Greenwell - Washington State, USA (netto to net to email me)

- "Transponders in Sailplanes - Feb/2010" also ADS-B, PCAS, Flarm http://tinyurl.com/yb3xywl

- "A Guide to Self-launching Sailplane Operation Mar/2004" Much of what you need to know tinyurl.com/yfs7tnz