A utility, boat, or cargo trailer spare lives rougher than most vehicle spares. It rides low, sits close to the road, and hangs in the spray zone behind your tow vehicle's rear tires; a boat trailer spare often gets dunked at the ramp on top of that. It is also often a different size than most tire-cover advice assumes: small utility and boat trailer spares often run well under what a Jeep or travel trailer needs, sometimes down in the 20 to 24 inch range. Here is what changes when the trailer behind you is not an RV, and how to measure a trailer spare using the same sidewall-code method that works on any tire.
Why does a trailer spare take more abuse than a vehicle's own spare?
A Jeep or truck spare usually mounts on a tailgate or under the bed, tucked at least partially out of the direct spray. A trailer spare does not get that luxury. Most utility, cargo, and boat trailers mount the spare on the tongue, a frame rail, or a small bracket near the axle, all low and close to the ground, right where a tow vehicle's rear tires kick up water, gravel, and road film mile after mile.
Boat trailers add a second layer: launching and retrieving means backing the trailer, and the spare with it, into lake or ocean water regularly. Saltwater is worse than fresh, since residue works into the cover fabric and speeds up corrosion on the wheel if it never gets rinsed off. Cargo trailers pick up their own hazards too, from gravel job sites to loose straps that can rub against a spare mounted low on the frame.
None of this makes a cover optional; it just means the cover is working harder than one on a passenger vehicle and is worth choosing to shed grime, not just block sun.
What size is a utility or boat trailer spare tire cover, usually?
Trailer tire sizing varies more than most people expect, mostly because trailer wheels get sized for load rating rather than ride comfort. Small utility trailers and bunk-style boat trailers commonly run 8 to 13 inch wheels, putting the mounted spare around 14 to 24 inches. Larger tandem-axle boat and cargo trailers usually step up to 14- or 15-inch wheels, landing closer to 26 to 30 inches, the same range most travel trailers fall into.
| Trailer type | Wheel size | Spare diameter | Fits 27-35in? |
|---|---|---|---|
| Small utility trailer | 8-12in | ~14-20in | Often too small |
| Boat trailer (bunk/roller) | 12-13in | ~20-24in | Sometimes not |
| Larger boat trailer (tandem axle) | 14-15in | ~26-29in | Usually fits |
| Cargo trailer (enclosed, car hauler) | 14-16in | ~27-30in | Usually fits |
| Travel trailer (for comparison) | 14-16in | ~27-30in | Usually fits |
Treat every number in that table as a starting point, not a final answer. Tread depth, tire brand, and load range shift the actual diameter by an inch or more. If your spare measures under 27 inches, know that going in: most off-the-shelf covers, including the sizes we carry, run from 27 to 35 inches, so a small trailer spare can fall outside standard sizing everywhere. Measure first, then shop.
Sized 27 in. to 35in. Works on a cargo or larger boat trailer spare as well as on a Jeep.
How do you measure a trailer spare tire for a cover?
The measuring method does not change just because the tire is on a trailer instead of a Jeep. Two ways to get your number, and either works.
Read the sidewall. Every trailer tire has a code printed on it. Metric sizes look likeST205/75R14, where the numbers break down into tread width, aspect ratio, and rim diameter. Named sizes like 4.80-12 already show the wheel diameter as the second number. Our tire cover size guide walks through the full formula and includes a calculator.
Measure directly. Stand the mounted spare upright and measure straight across the center, from the outer edge of the tread on one side to the other, in inches. This works even when the sidewall text has worn off, which happens more often on trailers than vehicles since the spare can sit unused for years.
Round up if you land between two sizes. A slightly large cover can be cinched down with its drawstring or elastic hem; one that is too small will not stretch over the tread.
What should you look for in a cover built for trailer duty?
Given everything a trailer spare sits through, prioritize three things over looks. A weather-resistant fabric that sheds water and road film rather than absorbing it, since a cover that stays damp invites mildew faster than one that dries quickly. A snug elastic or drawstring hem, since trailers ride rougher than the tow vehicle pulling them, so a loose cover works itself free faster than one on a Jeep. And an easy rinse: if your trailer picks up mud, lake water, or salt spray regularly, a cover you can hose off in a couple of minutes will actually get used.
Is a boat trailer cover different from a utility or cargo trailer cover?
Not in terms of sizing. A cover is sized by tire diameter, not by what the trailer hauls. A 24-inch cover fits any 24-inch spare, whether that trailer carries a boat, a mower, or a load of lumber.
Where trailer type matters is priority. Boat trailer owners get the most value from a fabric that rinses clean fast and resists mildew, since ramp dunking is part of the routine. Utility and cargo trailer owners tend to care more about abrasion resistance, since straps and gear near the spare can rub during loading. Either way, fit comes first, since a loose cover wears out fast no matter what it is made of.
One more note if you are shopping for a travel trailer or motorhome instead: those spares run larger on average and face more sun than road grime, since they usually sit higher and further from the spray zone. If that is your situation, our RV spare tire cover guide covers that side of things in more depth.
Upload a boat name, logo, or design. Made to order in your measured size, so measure first.
Frequently asked questions
Will a Jeep spare tire cover fit my utility trailer spare?
Yes, if the diameter matches. Covers are sized by tire diameter, not vehicle type, so a 28-inch cover fits any 28-inch spare whether it rides on a Jeep, a cargo trailer, or a boat trailer.
My boat or utility trailer spare measures smaller than 27 inches. What are my options?
Measure again to confirm the exact diameter, since small trailer spares are easy to misjudge. If it is genuinely under 27 inches, you are outside the standard size range most tire cover shops carry, ours included, so reach out before ordering the closest available size and hoping it stretches.
Does a trailer spare tire cover need to be waterproof?
It needs to be weather-resistant, not fully sealed. A completely airtight cover can trap moisture after a rainstorm or a trip to the boat ramp, which encourages mildew. Look for one that resists water and road grime but still lets the tire breathe.
How do I keep a trailer spare cover from working loose on the road?
Start with the right size, since an oversized cover is the most common cause of flapping. Seat the hem fully behind the tread edge before you tow, then check it again after your first long trip since elastic settles with use.
Find the Right Cover for Your Trailer Spare
Sized 27 in to 35 in on weather-resistant fabric, made to fit utility, boat, cargo, and travel trailer spares alike.
Shop Tire Covers →Not sure on sizing? The tire cover size guide takes about a minute.
0 comments