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Difference Between ST185/80R13 and ST185/80D13 For Picking Out Spare


My boat trailer currently has St185/80R13 tires on it. Would it matter if my spare tire was a ST185/80D13? No sure what the difference is.


Helpful Expert Reply:

The difference between the letter R and D in the tire size you mentioned has to do with the construction of the tire. R stands for radial and D stands for Bias Ply.

The main difference between a radial ply and a bias ply tire is how the tire is constructed. The cords inside a bias ply tire run at a 32 degree angle to the direction of travel, and the cords on a radial tire run at a 90 degree angle to the direction of travel, or across the tire from wheel lip to wheel lip.

A radial tire flexes more than a bias tire, giving it better ground contact, traction, stability, and tread wear.

A radial tire will normally run cooler than a bias ply tire, especially when the tire is under a load. A tire that runs cooler will last longer.

A bias ply tire has a stiffer sidewall than a radial, a feature that might be useful for off road applications like farm use.

At one time radials were more expensive than bias ply tires, but now they are closer in price. This makes radials a much better value.

To answer your question though yes, you could use a bias ply as a spare in an emergency situation to replace a radial. Since we don't have any radials in that tire size we don't offer a solution for a radial as well.

I attached an FAQ on trailer tires for you to check out as well.

helpful expert reply by:
Jameson C

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