EQUUS - - Eq In Brief -

Much of the in­for­ma­tion you need to eval­u­ate your tires, and use them safely, will be recorded on their sides in raised letters and num­bers.

DOT stands for De­part­ment of Trans­porta­tion; th­ese letters are fol­lowed by a string of letters and/or num­bers, which are the man­u­fac­turer’s codes. The last four dig­its of this string in­di­cate the week and year a tire was made. So, for ex­am­ple, the dig­its 1116 in­di­cate that the tire was man­u­fac­tured in the 11th week, or March, of 2016.

ST means “spe­cial trailer,” which is specif­i­cally de­signed for use on trail­ers. (LT tires are de­signed for use on light trucks; P in­di­cates the tire was de­signed for use on pas­sen­ger ve­hi­cles.) Th­ese letters will be large and fol­lowed by other num­bers, which in­di­cate the size of the tire. For ex­am­ple, 225/75R15 means that the tire is 225 mil­lime­ters wide, with a side­wall height that is 75 per­cent of the width. The R in­di­cates a ra­dial tire; a D (di­ag­o­nal) would mean that the tire is bias ply. The 15 refers to the di­am­e­ter of the rim, in inches.

Max load in­di­cates the max­i­mum weight a sin­gle tire was de­signed to sup­port. Usu­ally, you’ll see one or two num­bers in pounds. For ex­am­ple, a tire might be able to carry 1,480 pounds if used on a sin­gle axle trailer or up to 1,280 pounds if used on a tan­dem axle trailer. Mul­ti­ply the max load num­ber on each tire by the num­ber of tires on the trailer to de­ter­mine the max­i­mum weight you can tow. If your loaded rig is heav­ier than that, you’ll need to up­grade your tires.

The max psi, fol­lowed by a num­ber, such as 45, is the max­i­mum in­fla­tion pres­sure (in pounds per square inch) of the tire. You’ll need a good tire gauge to check and mon­i­tor the in­fla­tion of your tires. Just look­ing at the tires isn’t good enough. An­other handy tool is an air com­pres­sor that can be pow­ered by plug­ging into the cig­a­rette lighter of your tow ve­hi­cle.

A num­ber fol­lowed by “mph” is the tire’s max­i­mum safe speed. This in­for­ma­tion isn’t re­quired by the DOT, so not all tires show this. But most trailer tires are rated for high­way speeds of 65 mph. Main­tain­ing higher speeds for long dis­tances may cause tires to over­heat. Some tires may be rated for max­i­mum speeds des­ig­nated by letters: L in­di­cates a max­i­mum speed of 75 mph, and M is a max­i­mum speed of 81 mph.

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