Trailer tires usually fail from age and sun exposure, not mileage — a trailer that sits in a driveway most of the year can blow a tire with tread that still looks new, because the sidewall rubber dries out and cracks faster than it wears down. Check the manufacture date and sidewall condition before towing, not just the tread depth.
A trailer tire does a different job than the tires on the vehicle towing it. It's built to carry a static load and resist sway, not to grip the road the way a drive tire does, which is why trailer tires are rated "ST" rather than the passenger or light-truck ratings most people recognize. That difference in job is also why trailer tires fail differently: age and UV exposure crack the rubber well before the tread wears flat, especially on a trailer that spends most of its time parked rather than on the road.
That makes a pre-tow check worth doing even on a trailer that looks fine sitting in the driveway.
Why do trailer tires fail even with good tread left?
Sun and ozone exposure dry out the rubber compound from the outside in, and that process doesn't pause just because the trailer isn't moving. A tire that's been sitting in one spot under its own weight for months can also develop a flat spot or sidewall weakness that only shows up once it's back under load on the highway.
Should trailer tires be replaced by age or by tread depth?
Age first, tread second, is the safer order for a trailer tire. A tire with deep tread but an old manufacture date is a worse bet than one with moderate tread and a recent date, because the rubber's condition — not how much is left to wear down — is what's actually failing at highway speed.
Why does trailer tire pressure matter more than it seems to?
A trailer tire's sidewall is built stiffer than a passenger tire's, so it doesn't flex and visibly bulge when it's underinflated the way a car tire does. That makes it easy to tow for weeks on a tire that's low on air without noticing from the outside, right up until heat buildup from the extra flex causes a blowout on the highway. Checking pressure with a gauge before a tow matters more on a trailer than on the vehicle pulling it, because the trailer tire won't give you the same warning signs.
How do I find out how old my trailer tires are?
Look for the DOT code molded into the sidewall; the last four digits give the week and year the tire was made. If the trailer hasn't moved much, check this before a trip instead of relying on how much tread is left.
Do both tires on a trailer axle need to match?
Yes, especially on a tandem-axle trailer, where mismatched tread depth or tire size on the same axle can cause the trailer to pull or sway while towing. Replace tires in pairs on an axle when one fails rather than mixing an old tire with a new one.
What should I check before a long tow, not just before buying?
Check tire pressure cold, before the trailer has been sitting in the sun, and look over both sidewalls for cracking even if the trailer was fine on the last trip. A trailer that sits between tows needs this check more often than one that's used every week.