Tyres: load rating, LT vs P, and what upsizing actually costs you
You want to go up a size, put a decent all terrain on, and get on with the trip. Nobody at the counter is going to talk you out of that. What they may not spell out is that a tyre is a rated structural component holding up a two-tonne vehicle, and that going bigger changes your speedometer, your gearing, your braking and your fuel use all at once — mostly in the wrong direction.
The number that matters most is the load rating. The two tyres on an axle have to carry what that axle is rated to carry, at the pressure the tyre needs to achieve that rating. Get it wrong and you have a vehicle that's overloaded in a way nothing on the dash will tell you about. Everything else here is secondary to it.
What the numbers on the sidewall mean
A common Australian size reads something like 265/65R17 112S. From the left:
- 265 — nominal section width in millimetres.
- 65 — aspect ratio, the sidewall height as a percentage of the width. A 65 is a taller sidewall than a 55 on the same width.
- R — radial construction. Almost everything you'll buy is radial.
- 17 — rim diameter in inches. Yes, the size mixes metric and imperial.
- 112 — the load index. A code, not a weight.
- S — the speed rating. Also a code.
Some tyres carry letters before the width. LT means light truck construction. P is the North American passenger standard. Plenty sold here carry no prefix, which generally means passenger construction to the European metric standard.
You'll also see flotation sizing, written like 33x12.50R17: overall diameter, width and rim diameter, all in inches. That's what people mean by "33s".
Load index and speed rating are codes, not numbers
The load index translates to a maximum load per tyre in kilograms via a standardised published table. It isn't a percentage and you can't eyeball it. Ask the retailer for the kilogram figure for the exact size and index you're quoting. We're deliberately not printing a conversion table here — get it from the tyre manufacturer's own data.
The speed rating letter maps to a maximum sustained speed, also from a published table. It matters more than people assume, because chunky light-truck tyres often carry a lower speed rating than the road tyres the vehicle left the factory on.
In general terms, Australian rules expect replacement tyres to suit the vehicle's speed capability, and there are arrangements covering off-road tyres carrying a lower rating. The detail, including whether any label or notice is required, is set by the rules your state or territory applies. Check with your road authority, not a forum.
LT versus passenger construction
This is a genuine trade-off, not a quality ranking.
LT construction has a heavier casing, more material in the sidewall and tread, and runs at higher pressures. That's what gives it the load capacity and the resistance to sidewall damage from rock, gibber and stick country. The cost is weight, in the worst place — unsprung and rotating. Slower acceleration, longer braking, more fuel, harsher ride when empty.
Passenger construction is lighter, quieter, rides better, uses less fuel and has less load capacity at a given size. The sidewall is more vulnerable off road.
The honest rule: if the ute lives loaded, tows, or spends real time on sharp rock, LT earns its weight. If it's a work ute that sees the beach twice a year, LT is a lot of penalty for protection you rarely call on.
One thing gets missed constantly. A tyre's load rating is quoted at a specified inflation pressure. Fit LT tyres, keep running the placard pressures for the original passenger tyre, and you may sit well below the pressure that rating depends on. Ask what cold pressure your size needs for the weight you carry.
Load rating, axle ratings and GVM
Add up the rated capacity of the two tyres on an axle. That total has to cover what the axle is rated to carry, not just some share of GVM. Axle ratings are on the identification plate or in the manual, and on a loaded touring ute it's almost always the rear axle that's tight.
Two traps:
- A GVM upgrade does not upgrade your tyres. If a certified upgrade raises your legal loaded mass, the tyres have to support the higher axle loads at their rated pressures. Ask the certifier what tyre load rating the upgrade assumes.
- A big load index is not permission to load up. Tyre capacity is a ceiling to stay under, not a new allowance.
What upsizing actually costs you
Speedometer and odometer
A taller tyre covers more ground per revolution, so the vehicle turns fewer wheel revolutions per kilometre and the speedo reads lower than your true speed. Australian rules require a speedo not to under-read — a degree of over-reading is tolerated. Going taller pushes you toward the direction that isn't allowed, and it's your problem, not the tyre shop's.
The odometer goes the same way, which stretches service intervals and makes the trip meter you use for fuel range optimistic. Many vehicles can be recalibrated; ask before you buy, because on some it's easy and on others it isn't.
Effective gearing
A taller tyre is a taller final drive. Less torque reaches the ground in every gear, the auto hunts more, and it spends longer slipping the converter. That shows up as heat, and towing up a range or working soft sand is where it bites. Go a long way up in diameter and you may end up re-gearing the diffs, which is a much bigger job than a set of tyres.
Braking
A bigger rolling diameter gives the tyre a longer lever against the same brakes, so stopping distances go up, and heavier wheels add rotating mass on top of that. ABS and stability control were calibrated around the original rolling diameter, and nobody tells them you changed it. Same for adaptive cruise, lane keeping, AEB, trailer sway control and tyre pressure monitoring — they all read wheel speed.
Fuel
Heavier tyre, more rolling resistance, more aggressive tread, taller gearing, more frontal area if you went wider. All of it pushes the same way. Expect a measurable increase, and factor it into range planning for remote trips.
Rim width, offset and clearance
- Every tyre has a range of rim widths its manufacturer approves. Outside it, the tyre's shape and rating aren't what the data says.
- Offset changes track width. Pushing wheels outward alters steering feel, loads bearings differently, and can put tyre outside the guard line — a roadworthy matter, and state rules vary.
- Clearance needs checking at full lock, at full suspension compression, and at both together. Guard liners, sway bar links and castor rod bushes are the usual casualties.
- Increased tyre diameter counts toward your state's lift allowance alongside any suspension lift. Our suspension guide covers that.
The spare, and pressures
Whatever you fit, the spare has to match. On a full-time four wheel drive with a centre diff, a spare of a different rolling diameter loads the driveline continuously, and that's an expensive way to learn. Check the new size fits where the spare lives, too — under-body winch cradles, tailgate carriers and rear bar swing-outs all have size limits.
Placard pressures assume the standard tyre at a standard load. Once you've changed the tyre and added a bar, drawers and a canopy, they're a starting point at best.
Airing down for sand, corrugations and rock is one of the highest-value skills in touring, and it needs a compressor and a decent gauge in the vehicle, not at home. Air back up before the bitumen. An under-inflated tyre at highway speed under load builds heat in the sidewall, and heat destroys tyres.
Before you buy
- Find your GVM and your front and rear axle ratings from the identification plate.
- Get the kilogram load figure for the exact tyre and index in writing, and check two tyres cover each axle rating.
- Ask what cold pressure that rating depends on, and whether it suits how you load the vehicle.
- Check the speed rating against your vehicle, and your state's position on lower-rated off-road tyres.
- Confirm the retailer lists the size and offset for your series and build date, and quote your VIN — guard clearance and wheel fitment change between series.
- Ask whether the speedo can be recalibrated for the new diameter, and what that costs.
- Check the spare will still fit its carrier, and buy a matching one.
- If you're lifting too, check the combined lift plus diameter increase against your state's rules first.
- Budget for a compressor, a gauge and a plug kit at the same time. Not accessories — the tyre's support crew.
This guide is general information, not fitting or engineering advice. Fitment, compliance and safety depend on your specific vehicle — confirm with the retailer and a qualified fitter before you buy.