Look closely at your tyre's sidewall and you'll spot a size like 225/45R17, followed by something easy to overlook: a number and a letter, such as 91V. That combination is the load index and speed rating, and it matters just as much as the size itself when it comes to safety and roadworthiness.
What the load index tells you
The load index is a number, usually somewhere between 70 and 126 for passenger vehicles, that translates to a maximum weight each tyre can carry when correctly inflated. A load index of 91, for example, corresponds to a maximum load of 615kg per tyre. Fit a tyre with a lower load index than your vehicle requires and you risk overloading it, which can lead to excessive heat build-up, faster wear, and in worst cases, tyre failure at speed.
It's worth remembering that this maximum load rating only applies when the tyre is inflated to its recommended pressure. Run your tyres under-inflated and their effective load-carrying capacity drops, even if the sidewall number stays the same. That's one more reason a monthly pressure check is worth the five minutes it takes.
What the speed rating tells you
The letter that follows the load index is the speed rating, showing the maximum speed a tyre can sustain under controlled test conditions. Common ratings for everyday cars include S (180km/h), T (190km/h), H (210km/h), V (240km/h) and higher-performance letters beyond that. Utes and 4WDs often carry ratings like Q or R, reflecting their load-focused design rather than outright speed.
Higher speed ratings generally mean firmer sidewalls and compounds engineered to handle heat better at sustained speed, which can also translate to sharper handling. That doesn't mean a higher rating is always better for your situation. A ute doing school runs and gravel driveways benefits more from load capacity and durability than a rating built for autobahn speeds.
Where to find your vehicle's requirements
Every vehicle has a tyre placard, usually found on the driver's door frame, glovebox, or fuel filler flap, listing the manufacturer's minimum load index and speed rating for that model. Under Australian Design Rules, replacement tyres need a load index equal to or higher than what's specified on the placard. Speed ratings have a little more flexibility: the general guidance is that a replacement shouldn't be rated below 140km/h for a 4WD or 180km/h for a passenger vehicle, even if the placard technically allows for less.
Mixing ratings across an axle
As a general rule, avoid fitting tyres with different load indexes or speed ratings on the same axle, and ideally keep all four matched. Mismatched ratings can affect handling balance and, in some cases, may not meet compliance requirements for a roadworthy certificate.
Why it's worth checking before your next tyre change
It's easy to just ask for the same size when you're due for new tyres, but the load index and speed rating deserve equal attention. Fitting a tyre that's under-rated for your vehicle isn't just a compliance issue, it's a safety one, particularly if you're towing, carrying a full car of passengers, or covering long stretches of highway.
If you're not sure what your vehicle's placard requires or want a second opinion before fitting new rubber, the team at Global Tyres Geelong in North Geelong can check your placard specs against any tyre you're considering and make sure what goes on your car is properly rated for the way you actually drive.