Tyre size: upsizing and downsizing. How does tyre sizing work and how to I measure this? If you are a little bit unsure as to how tyre sizing works and how to measure your tyre size you have come to the right place. Your tyre size is individual to your vehicle and the manufactures specifications. You can find this on your tyre sidewall, tyre placard, in your owner’s manual, online or by using a tyre size calculator. Ultimately, there are so many tyre and wheel options it can be confusing at times. Motorists really are spoilt for choice. Give us a call today to discuss tyre sizing, specific to you and your vehicle with the experts.

Tyre specifications and guidelines

Your vehicle is only suited to a select few tyre size combinations which are ideal for safely travelling on the road. If steering away from the vehicle manufactures nominated tyre size, keep in mind your vehicle could become un-roadworthy. Size is not the only thing to consider when selecting a tyre. It is important to be aware of load, tread, tyre design, performance, PSI and much more.

How to measure your tyre size?

You can find the sequence of numbers and letters indicating your tyre size on your existing tyre on its sidewall, search engines, by using a tyre size calculator, in your owners manual or tyre placard.

The letter at the beginning of the sequence generally indicates the type of vehicle such as passenger or light truck. The following three numbers suggest tyre width in millimetres from sidewall to sidewall. The following two numbers suggest aspect ratio and construction. Aspect ratio is the height of the tyres cross-section to its width. The letter is symbolic of construction meaning the letter ‘R’ stands for radial so that the layers run radially across the tyre. The following number indicates wheel diameter. Wheel sizes are defined by diameter x width. This number will tell us the wheel size the tyre was intended to fit. The end of the sequence is symbolic of load and speed rating. You can find a load and speed index table online.


It is safe to minimally upsize your tyre if you were looking for wider tyres which fit larger diameter wheels. However, you must still meet your vehicle manufactures guidelines. Plus sizing your tyre is pretty common when aesthetics are at play. Larger tyres and wheels are quite distinctive and appear dominating. Many motorists choose to upsize tyres as it can improve performance such as steering and braking, grip and ride comfort. However, you may be compromising on fuel economy and acceleration. The maximum increase in combined wheel and tyre diameter allowed in Queensland is plus 15mm or minus 26mm. Within those limits, any rim diameter is allowed. You can find a tyre plus sizing calculator online or speak with the experts at Bud’s Tyres to match your performance goals.


Downsizing tyre size is quite uncommon in Australia. You must also pair your tyre with the appropriate wheel size. Attempting to fit a narrow tyre onto a wheel by forcing the sidewalls to stretch wider than they are designed for is what you should be avoiding. The most common reason and individual may downsize is if they are after narrow and smaller snow tyres.

The law

When alternative tyres and wheels are fitted to your vehicle you must follow local legislation and guidelines. It is also crucial that you contact your insurance provider beforehand. This is so that any possible future claims are not rejected as a result of vehicle modification. If your vehicle is under warranty is may also be a good idea to check with your manufacturer.

Significant modifications need to be assessed by a licensed certifier. Strict standards are in place to specify allowable combinations of wheel and tyre sizes. Changes to wheel and tyre size may alter your vehicles handling and performance and must be regulated. You can find the wheel and tyre combinations recommended by your vehicles manufacture on your tyre placard. You must not fit wheels with rim widths less than the minimum width fitted by the vehicle manufacturer for the particular model.


  • The alternative wheel must not increase wheel track of passenger cars (or derivatives) by more than 25 mm beyond the maximum specified by the vehicle manufacturer. The wheel track of off-road four wheel drive vehicles and goods vehicles must not be increased by more than 50mm beyond the maximum specified by the vehicle manufacturer for the particular model.
  • Where non-original axle or suspension components are fitted, the wheel offset in relation to the axle or stub axle assembly used shall not be increased by more than 12.5 mm each side of the vehicle based on the specifications for the axle components.
  • The wheel and tyre must be contained within the bodywork or mudguards, including any flares, when the wheels are aligned straight. The wheel and tyre must not contact any part of the body or suspension under all operating conditions, including when the front wheels are steered to full lock with the suspension fully compressed.
  • All wheels fitted to an axle must be of the same construction, diameter, offset, width and mounting configuration, except for spare wheels used in an emergency situation. The wheel must not prevent the wheel nuts from fully engaging their studs.
  • The wheel must be designed for the particular hub/axle in respect to bolt pitch circle diameter and wheel nut tapers. Wheels with slotted stud holes must not be used.
  • Speedometer accuracy must be maintained and adjusted where necessary.
  • Wheel spacers or adaptors must not be used for wheel conversions between the wheel mounting face and the wheels unless fitted as original equipment by the vehicle manufacturer.

Which tyre size is right for me?

Information regarding plus size tyres can be overwhelming and contradicting. When considering alternative wheels and tyres for your vehicle, it is best to talk to the team at Bud’s Tyres to guide you. Each circumstance is unique including vehicle type and application hence why it is best to talk to the experts.

Contact Info

Opening Hours

Mon - Fri: 8:00 AM - 5:00 PM
Saturday: 8:00 AM - 1:00 PM