How Tyre Sizes and Speed Ratings Actually Work

From Tyre Size to Load and Speed Ratings, Here's What You Need to Know to better understand your tyres

Every tyre sold has a size code moulded into its sidewall, something like 205/55R16 91V. It looks cryptic at first, but each segment tells you something specific about the tyre's dimensions, capacity, and performance limits.

Breaking Down the Size Code

Using 205/55R16 91V as an example, here's what each part means, based on the standard breakdown used by manufacturers like Continental:

• 205: the tyre's width in millimetres, measured from sidewall to sidewall.
• 55: the aspect ratio, meaning the sidewall height is 55% of the tyre's width.
• R: radial construction, which is standard on virtually all modern passenger tyres.
• 16: the wheel rim diameter in inches that the tyre is designed to fit.
• 91: the load index, a code corresponding to the maximum weight the tyre can safely carry when properly inflated.
• V: the speed rating, showing the maximum speed the tyre can sustain at that load.

What the Load Index Actually Means

The load index is a number, typically ranging from around 75 to 120 on passenger vehicles, and each figure corresponds to a specific maximum weight in kilograms per tyre. According to Michelin's guide to load and speed ratings, vehicle manufacturers use this figure to determine which tyres are appropriate for a specific vehicle, factoring in the car's weight, passengers, and cargo.

It's legal and generally fine to fit a tyre with a higher load index than what's specified for your vehicle. What you should never do is fit a tyre with a lower load index than your vehicle's manufacturer specifies, since that risks the tyre being overloaded during normal use.

What the Speed Rating Actually Means

The speed rating, shown as a single letter at the end of the code, indicates the maximum speed the tyre can sustain while carrying its rated load, under the manufacturer's test conditions. 

A few of the most common ratings you'll encounter on passenger vehicles in South Africa:

• S : rated to 180 km/h
• T : rated to 190 km/h
• H : rated to 210 km/h
• V : rated to 240 km/h
• W : rated to 270 km/h

It's important to understand that a higher speed rating doesn't mean you should drive faster than the legal limit. What it does affect is heat dissipation, high-speed stability, and overall grip characteristics, even well below the tyre's maximum rated speed.

This is part of why fitting a lower speed-rated tyre than your vehicle's manufacturer specifies isn't just a performance downgrade, it can genuinely affect handling and safety.

Why This Matters When Replacing Tyres

• Always match or exceed the original load index and speed rating specified for your vehicle, found in your owner's manual or on the placard inside the driver's door.
• Mixing tyres with different speed ratings on the same vehicle isn't illegal in all cases, but it's not recommended, since it can create uneven handling characteristics between axles.
• Insurance claims can, in some circumstances, be affected if a tyre-related incident is linked to fitting tyres with a lower specification than the manufacturer requires.

Understanding your tyre specifications also matters when it comes to preventing failures altogether. If you want to know what to do if things go wrong on the road, our guide on what to do if your car tyre bursts covers the immediate steps to take.

Where to Find Your Vehicle's Correct Specification

Your vehicle's correct tyre size, load index, and speed rating are listed in your owner's manual and on a placard, usually found on the inside of the driver's door frame or the fuel filler flap.

This is the specification to work from, not the numbers on your current tyres, particularly if a previous owner may have fitted non-standard tyres at some point.

The Bottom Line

Tyre size codes look complicated, but they break down into a handful of straightforward figures once you know what to look for.

When it's time to replace your tyres, match your vehicle's specified load index and speed rating exactly, or go higher, but never lower. It's one of the simplest ways to keep your vehicle handling the way the manufacturer intended.

Sources & Further Reading: 

Continental: Tyre Markings Explained
Michelin: Tyre Load Rating & Speed Rating Explained