Comparable tire sizes and retail options

If your tires still have the old "inch sizing" they are too old to safely use for anything more than rolling it around the driveway. Modern tires have a metric DOT (Department of Transportation) tire sizing system. But first we need to talk about what you actually need, and why. It would be best to not purchase anything until you figure out the answers to some of these bullet points. The topic of trailer tires/wheels is a large one, A VERY LARGE ONE, and it is very likely the most misunderstood topic in trailering today.

I'm not answering any of your questions, and I'm raising more questions that need answered so you can confidently place your order knowing the parts will fit your trailer needs.

*Types of Tires, LT versus ST tires. LT stands for Light Truck, it might not be the best choice for a trailer tire, but millions of trailers are running LT tires. Some tire companies like Michelin, don't even offer a ST tire, and simply use their LT tires. I have towed our single axle race car trailer tens of thousands of miles with LT tires for twenty years without any issues. ST stands for Special Trailer. ST tires are built with stiffer sidewalls which is important on tandem axled trailers which have higher side loads.

*Weight Ratings, You need to know what the Gross Weight "fully loaded" is of your trailer. This will determine the proper weight rating of the tires AND the weight rating of the wheels. Most car wheels are rated at 1970# maximum, most trailer wheels are rated at 3050#. The trailer wheels have a thicker, heavier wheel center. Wheels do wear out over time. Most are damaged by the knuckleheads at the tire shops with their impact guns. Each time the wheel is reinstalled, and "over torqued" the taper on the lug hole gets larger. At some point your lug nut is bottoming out on the hub and not actually tightening the wheel. As you drive the wheel wears the lug holes until the wheels falls off. Losing a wheel, or even a tire tread on a trailer is going to do a lot of damage under the trailer. Use Trailer Wheels, and Trailer Tires properly rated for your application. Use a real torque wrench to tighten your lug nuts. Too little is bad, and too much torque is bad, either is unsafe.

*Speed Rating If you look up most trailer tires, they are only rated for speeds under 60 miles per hour. It's a joke that they would even make something rated so slow. You know that number is going to be abused. Just think how much heat they need to withstand when your towing home on a Sunday afternoon across the 120-degree desert? The speed rating is molded into the tire sidewall. The difference in cost to run "premium trailer tires" versus the cheap "house brands" is minimal. Good Year has vastly improved their ST trailer tires and the top model line is now called Endurance, NOT the Marathon Tires. Maxxis makes a premium ST tire called the M 8008 in many different sizes and weight ratings. Both are available direct from Discount Tire Direct, or even Wall Mart online. Most companies will ship with free freight on orders over $100 minimum purchases right to your residential door. Your local tire independent store will mount the new tires on your wheels, balance the tires, and cover the old tire disposal for $20/$25 per tire. Doing the replacement this way will save you a lot of money over taking your trailer to a tire store and having them destroy your trailer. You need to be able to remove and reinstall a tire on the side of the road during an emergency, so this gives you a good time to practice in your driveway.

*Tire Dates. Trailer tires do not wear out, they age out. About the only way to wear out a trailer tire is if the axle is bent out of toe alignment. You will get lots of different ideas on how long the useable life span is but take my word, five years is a good rule of thumb, and seven years is the absolute maximum useable life. All modern tires are required to have date codes molded into the side wall. Do not buy used trailer tires for highway use. You can google DOT tire date codes for how to read the code. If you end up buying your tires locally, tell the tire store, you will not accept "old date code" tires. Put them "on notice", be an informed consumer.

*Bias Ply versus Radial Ply trailer tires. I think it's best for most of us to use the cooler running Radial Tries. The only downside is that they have a slightly wider sidewall profile which can make mounting the wheels difficult on some style trailers with limited fender skirts.

*Air Pressure, run the maximum "cold" air pressure that is molded into the tire sidewall. For example, on my 235/75R/16 tires they run at 80# PSI. Tires will run at a very high temperature if you run them "underinflated" and it can damage the tire.

*Hub Caps, your old original hub caps will likely not fit your new style trailer wheels. Some of the "spoke type" trailer wheels sold by E-Trailer have no provision to mount any hub cap. You can find modern reproduction hub caps and beauty rims that fit newer wheels at Hub Cap Mike in Orange, CA, or online.

*Wheel rim size, rim width?

*Wheel Bolt Circle, how to measure?

*Wheel Center Hole size? Wheel mounted by Hub Centric, or Lug Centric method?

*Wheel Back Spacing, how to measure? Most trailer wheels have zero back space difference, that's not true with most car wheels.

*It is best to not run rubber "car" air valves. Any auto parts store has metal air valves for about $5 each, they are fully metal and secured in the wheel with a nut. This is very important on the higher air pressures.

* Information Resource Centers Dexter Axle Company, Dexstar Wheels, E-Trailer, Good Year, Maxxis Tire

This is quite a bit of information YOU NEED TO KNOW. Do you think the kid at the local tire store knows any of these topics, or wants to learn anything more than selling you some tires and getting your trailer out of his driveway? You will find lots of this information on the internet; your biggest challenge will be separating the facts from all of the fiction. And you're not going to get the straight answers to these technical questions on Face Book.

Good luck with your project.

John Palmer

Just to mention, wheel selection can be a can of worms as well. Factors like width and offset come into play.

Your old wheels may be ugly but if otherwise undamaged they can be refinished. Sandblasting and paint go a long way. Plus, you know they fit.

Comparing old tire sizes to new is going to be a tradeoff. Modern tires are all wider or shorter than the old ones. You'll need to do some homework to determine if you can go wider and keep a close match to diameter or if you need to stay close on width and can live with the smaller diameter.

Just for fun, a 700-15 would have a metric size of 175-100-15.. A size that doesn't exist.   That 100 in me middle is called aspect ratio and is determined by  dividing the Sidewall height by the tire width.  Back in the bias ply days,  aspect ratio was almost always 100. Nowadays 85 is about the highest you'll find. 

It gets to be even more fun when working with alpha numeric tire sizes. 

I have the 15” Kelsey Hayes original wheels on my 58 Shasta. Had Bias ply 7.00 x15 tires.

Found 205 90 R15’s from Power King. They are a Radial E load tire. Very good tire and very close to original tire I took off. Bought them at Cooper Tires.

Very good, and important discussion. Too many people do not pay attention, or understand this stuff. John Palmer, you are a never ending source of knowledge, hopefully making everyone safer. Thank you.