Trailer Tires to Replace P165/80-13 and B78-13 Tires on a Boat Trailer

Updated 02/09/2016 | Published 02/08/2016

Products Featured in This Question

Loadstar ST175/80D13 Bias Trailer Tire - Load Range D

(3888)

$93.94

Karrier KR03 ST175/80R13 Radial Trailer Tire - Load Range D

(2348)

$104.94

Question:

Need to replace boat trailer tires. Old set 1: P165/80R 13 Old set 2: B78 13 ST Estimated boat weight: 2500 - 3000 lbs. ? What do above tire #s mean?

asked by: Bill M

Expert Reply:

First, if a tire starts with a P like your P165 tires that indicates that they are passenger vehicle tires and should not be used on a trailer. This is because trailer tires are built with a thicker sidewall to handle more vertical load and passenger vehicle tires are not.

The other size uses the old designation. The ST in the B78 size indicates Special Trailer so these are trailer tires although old since that designation isn't really used much any more. The metric equivalent to the B78-13 is going to be an ST175/80-13.

The 175 is the section width of the tire in millimeters. The 80 is the aspect ratio or how much of the sidewall there is relative to the section width. And the 13 is the wheel size it fits.

The only thing left to figure out is the capacity. Based on the weight of the boat I recommend tire # AM10210. This tire is rated for 1,610 pounds at 65 psi so 2 of them would have a total capacity of 3,220 pounds.

Now if you have 2 axles you might be able to get away with a lower capacity tire but you might as well stick with the higher capacity because they will last longer. Also be sure to always inflate trailer tires to the maximum psi indicated on the tire. If under inflated they can generate excessive heat and lead to failure.

expert reply by: Michael H