How it's worked out
Tyre capacity = load per tyre (from the load index) × tyres on the groundCheck: tyre capacity ≥ GTM (and ideally ≥ ATM)GTM is the mass the loaded trailer puts through its own tyres when it's hitched to the tow vehicle. That's the load the tyres actually carry, so their combined rating must cover it. ATM includes the towball download as well, so sizing the tyres to ATM gives a margin for a van loaded unevenly or a heavier towball download than planned. If you leave GTM blank, the calculator estimates it as ATM minus 10%.
Worked example
A single-axle caravan with a 2,200 kg ATM and a 2,000 kg GTM runs two 235/75R15 104S tyres. Load index 104 is 900 kg, so the pair can carry 1,800 kg. That's 200 kg short of the GTM. The van needs tyres of at least load index 108 (1,000 kg each) to cover its GTM, and 112 (1,120 kg each) to cover its ATM.
Tyre load index chart
Maximum load per tyre in kilograms for common caravan, trailer and light truck load indexes.
| Load index | kg per tyre |
|---|---|
| 88 | 560 |
| 89 | 580 |
| 90 | 600 |
| 91 | 615 |
| 92 | 630 |
| 93 | 650 |
| 94 | 670 |
| 95 | 690 |
| 96 | 710 |
| 97 | 730 |
| 98 | 750 |
| 99 | 775 |
| 100 | 800 |
| 101 | 825 |
| 102 | 850 |
| 103 | 875 |
| 104 | 900 |
| 105 | 925 |
| 106 | 950 |
| 107 | 975 |
| Load index | kg per tyre |
|---|---|
| 108 | 1,000 |
| 109 | 1,030 |
| 110 | 1,060 |
| 111 | 1,090 |
| 112 | 1,120 |
| 113 | 1,150 |
| 114 | 1,180 |
| 115 | 1,215 |
| 116 | 1,250 |
| 117 | 1,285 |
| 118 | 1,320 |
| 119 | 1,360 |
| 120 | 1,400 |
| 121 | 1,450 |
| 122 | 1,500 |
| 123 | 1,550 |
| 124 | 1,600 |
| 125 | 1,650 |
| 126 | 1,700 |
Other limits on the running gear
- Wheels and rims. Rims carry their own load rating, often stamped on the rim. It needs to cover the same load as the tyre.
- Axle and suspension. The axle and springs have ratings too, and they can be lower than the tyres. The trailer's GTM is meant to stay within all of them.
- Brakes. Brake rules go by GTM as well. See what your trailer needs with the brake and safety chain checker.
Common questions
- What load rating do caravan tyres need?
- Together, the tyres must be able to carry the weight on them. When the caravan is hitched that's its GTM, the ATM minus the towball download. Divide the GTM by the number of tyres to get the minimum per tyre. Many caravan builders and tyre retailers go further and size the tyres to carry the full ATM, which leaves a margin for uneven loading and the moment the van is unhitched.
- Where do I find the load index on a tyre?
- It's moulded into the sidewall straight after the size, just before the speed letter. In 235/75R15 104S, the load index is 104 and the speed rating is S. Load index 104 means 900 kg per tyre.
- What do two load numbers like 121/118 mean?
- Light truck (LT) tyres often show two numbers. The first is the load index when the tyre runs on its own (single), the second when it runs in a dual pair. Caravans and trailers run single tyres, so use the first number.
- Does the spare tyre count?
- No. Only the tyres on the ground carry the load. The spare should have the same load rating as the others, so it's safe to fit.
- Is the tyre's load rating at any pressure?
- No. The load index is the maximum load at the tyre maker's maximum rated pressure. Run the tyre at a much lower pressure and it can't safely carry its full rating. Check the tyre placard on the caravan or the tyre maker's load and pressure table for the right pressure at your weight.
Sources
- Vehicle Standards Bulletin 1 (Revision 6): trailers with an ATM of 4.5 tonnes or less (August 2026), Australian Government, Department of Infrastructure
- Light trailer requirements: compliance with VSB1, Transport Tasmania
Rules checked against these sources on 12 October 2026. See rules, sources and method.