What Does the 75D Mean in a Trailer Tire Size ST205/75D15

Updated 07/06/2016 | Published 07/05/2016

Question:

Whats the 75d mean. Need it for a trailer two axle 18 foot

asked by: Mike V

Expert Reply:

In a tire size the second number is the aspect ratio. The aspect ratio is the percentage of the sidewall height relative to the width of the tire. The width is the first number; in this case for # AM3S640DX the width is 205 mm. The sidewall is 75 percent of 205 mm.

The D is indicating the tire is a bias ply tire as opposed to a radial tire. The difference is in the orientation of the plies or belts in the tires' construction. On a radial they run straight across the tire whereas in a bias ply they run at an angle.

So if the size you have now is ST205/75-15 load range C and the bolt pattern of the wheels you have now is 5 on 4-1/2 then # AM3S640DX will work for you. It is rated for 1,820 pounds at 50 psi. I have included some helpful links on trailer tires and wheels for you as well.

expert reply by: Michael H