Difference Between ST225/75D15 and ST225/75R15 Trailer Tire

Updated 11/10/2019 | Published 11/09/2019

Question:

What is the difference between a ST225-75D 15 and a ST225-75R? I know that the R is a Radial Tire. I have a 30foot travel trailer that takes ST225-75D tires, that is in need of tires. Dry weight is 7,500 pounds. Average weight is around 8,500 pounds. I have a 20foot cargo trailer that has ST225-75R tires on it. I believe they are 8 or 10 ply. Same rims, can I swap them out?

asked by: Leland B

Helpful Expert Reply:

The "D" designation that you see on the ST225/75-15 tires for your travel trailer indicate a bias ply tire, as opposed to the radial ply tires on your cargo trailer. The primary difference between bias and radial tires is how they're actually constructed, and more specifically the way that the cords run inside the tire.

On a bias tire the cords run at a 32 degree angle at the direction of travel while the cords of a radial tire run at a 90 degree angle to the direction of travel. This is just a technical way to say that a radial tire is going to be able to flex more, have better contact and stability, and because of this is generally the better option for highway driving. It's going to run cooler as well which makes it better for higher speeds.

A bias tire is going to be a bit stiffer at the sidewall than a radial. This makes it the better option for off-road/agricultural use.

With all this in mind, you can absolutely replace the bias tires on your travel trailer with the radials from your cargo trailer. In fact, assuming that you aren't taking the travel trailer down too many dirt roads and mountainous terrain, this would be the preferred setup.

Just note that you will want to change all the tires to radial on the travel trailer. A mix between radial and bias tires will result in uneven tread wear and a shortened tire lifespan. For a great ST225/75R15 radial tire I recommend the Provider # TTWTRTM2257515D, which features a maximum load of 2,540 pounds at 65 psi and an excellent speed rating of 81 mph.

I have included a short video review below on this tire that you can check out, along with a couple articles on trailer tires that you might find interesting.

expert reply by: Chris R