Trailer Tire Tread Depth: How Much Is Enough
Short answer: Treat the built-in wear indicators as the absolute floor for a trailer tire. Federal standards set those indicators at 1/16 inch on covered tires, but a towed trailer should be replaced well before the indicators appear. ST trailer tires are covered by a separate federal standard and are not held to the same passenger-car rules, so give them more margin. Check tread, pressure, and age before every long trip.
What the minimum really means
Federal safety standards use tread wear indicators as a built-in warning that a tire has reached the end of its useful tread. Standard No. 129, covering new non-pneumatic spare tires, requires an indicator that provides a visual indication when the tire has worn to a tread depth of 1/16 inch. Standard No. 109, covering older passenger car and spare tires, also requires a tread wear indicator. When the indicator bars are level with the surrounding tread, the tire is at that floor. The tread wear indicators on a trailer tire work the same way.
For an ST trailer tire, 1/16 inch is not a good depth for towing. A trailer tire spends its working life carrying a load, and it does not get the steering and braking relief that a driven axle gets. Treat the indicators as the absolute minimum, not as a service depth. If your trailer tires are at the indicators, they are already past the point where a loaded trailer should be on the road.
ST trailer tires are a different category
ST stands for special trailer, and ST tires are regulated differently from passenger car tires. Federal Motor Vehicle Safety Standard No. 119 applies to tires for special-use trailers, including ST tires, FI tires, and small-rim trailer tires. Federal Motor Vehicle Safety Standard No. 139, the newer standard for radial tires on light vehicles, specifically excludes special trailer tires in highway service.
That difference matters when you compare tread depth. A passenger tire that is acceptable for highway use is not assumed to be interchangeable with a trailer tire. The ST rating tells you the tire was built with trailer duty in mind, including load capacity and speed rating for towing. For more on the distinction, see ST vs LT trailer tires and the trailer tire maintenance guide.
Why trailer tires need more tread
Tread depth is not only about wet pavement grip. It is also the rubber layer that protects the casing from cuts, impacts, and heat. A trailer axle is often loaded near its rated limit, and the tires run at highway speed for hours without the load shifts that occur on a tow vehicle. That is harder duty than a typical passenger tire sees.
A passenger tire can be run to the wear bars because it is on a lighter vehicle with brakes and steering that help manage the load. A trailer tire has much less of that help. The same remaining tread that seems fine on a sedan can be too thin on a trailer, especially if the trailer is under-inflated or overloaded. For that reason, give trailer tires more margin and check load range for towing before you set the axle weight.
How to check tread depth before a trip
Use a tread depth gauge on the center of the tread and on both shoulders. Trailer tires often wear unevenly: the center can wear faster with overinflation and the shoulders can wear faster with underinflation. The tread depth measurement page explains how to read the gauge correctly.
Check the wear indicators in the main grooves. If they are level with the tread blocks, the tire is at the 1/16 inch floor. For towing, treat that as an automatic replacement signal. Also look for cracking between tread blocks, sidewall cracks, bulges, and cuts. The full pre-trip inspection checklist is worth running before any long tow. Properly inflated tires are safer and last longer, so set the cold inflation pressure to the trailer manufacturer's recommendation, not the maximum number molded on the sidewall.
- Tread depth gauge: check the center and both shoulders.
- Wear indicators: look for bars level with the tread in the main grooves.
- Sidewall and casing: look for cracks, cuts, bulges, and dry rot.
- Air pressure: set cold inflation to the recommended pressure.
- Age: check the DOT date code and replace damaged or aged tires.
| Check | What to look for |
|---|---|
| Tread depth | Wear bars level with the tread means the tire is at the 1/16 inch floor; replace before towing |
| Tread wear pattern | Center wear suggests overinflation; shoulder wear suggests underinflation; uneven wear needs attention |
| Sidewall | Look for cracks, bulges, cuts, or dry rot |
| Pressure | Set cold inflation to the recommended pressure, not the maximum molded on the sidewall |
| Age | Check the DOT date code and replace tires that are cracked or aged even if tread remains |
Don't forget age and storage
Trailer tires are often ruined by age and storage before they wear out from miles. A tread depth gauge tells you only how much rubber is left; it does not tell you whether the rubber is still sound. Check the DOT date code and look for age and dry rot on the sidewall and between the lugs.
If a trailer has been parked for months, inspect the tires before the first trip of the season. A tire that looks full depth can still be unsafe if the casing has cracked or the sidewall has deteriorated. Replace any tire that shows damage, regardless of its remaining tread.
What to pick for your trailer
Tread depth is one part of the trailer tire decision. Size, load range, speed rating, and construction matter just as much. For most trailers, an ST tire is the right family because it is built for trailer service. Some heavy trailers use LT tires, and the choice depends on the axle load and the trailer manufacturer's recommendation.
When you compare replacement tires, use the trailer tire buying guide to look at spec-based listings, and keep the same tire on both sides of an axle. The table below summarizes what to pick for common trailer uses.
What to pick for your use
| If you | Pick | Buying guide |
|---|---|---|
| You tow a boat or utility trailer short distances | Replace any tire at the wear indicators before towing; keep the pressure at the recommended cold setting | Best Trailer Tires in 2026: 15 Picks by Specs |
| You tow a travel trailer on long highway trips | Replace before the indicators appear and choose ST tires with a load range that covers your axle rating | Best Carlisle Tires in 2026: 14 Picks Compared on Specs |
| Your trailer sits for months between uses | Inspect age and sidewall condition first, then check tread; replace cracked or aged tires before the season | Best Kenda Tires in 2026: 15 Picks Compared on Specs |
| You need tires for a horse or livestock trailer | Use trailer-rated tires and check pressure before every trip | Best Free Country Tires 2026: 14 Picks by Specs |
| You tow heavy equipment at highway speeds | Choose a higher load range and replace tires before the tread reaches the floor | Best Travelstar Tires in 2026: 15 Picks by Specs |
Questions
What is the minimum tread depth for trailer tires?
Use the built-in wear indicators as the floor. In federal standards, the indicator is set at 1/16 inch on covered tires. For towing, replace before the indicators appear because trailer tires need more margin than passenger tires.
Why do trailer tires need more tread than passenger tires?
Trailers carry heavy loads without the steering and braking help of a tow vehicle. Deeper tread gives more rubber for heat, cuts, and load. Passenger-car minimums do not account for trailer duty.
Are ST trailer tires required to meet FMVSS 139?
No. FMVSS 139 excludes ST tires in highway service. ST trailer tires are covered under FMVSS 119 as specialty trailer tires.
Can I measure trailer tire tread with a penny?
A penny test gives a rough check on passenger tires, but a tread depth gauge is the better tool for trailers. Check the wear bars across the whole tread and inspect the sidewall before a trip.
How often should I check trailer tires before a trip?
Check cold tire pressure and do a visual inspection before every tow. Before a long trip, also check tread depth, sidewall condition, and the DOT date code.
Revision notes
- : First published.