Tyre Size Calculator
Tyre Size Calculator
Understand Your Numbers
Thinking about changing your tyre size or moving to a larger rim? Get it wrong and your speedometer lies to you, your gearing suffers, and your safety is compromised. Get it right with this guide.
Shop Tyres by SizeWhat Does a Tyre Size Mean?
Every tyre has a three-part size code moulded into its sidewall. Take 205/55R16 as an example — each number tells you something specific and critical about the tyre's dimensions.
The 205 is the tyre's width in millimetres, measured across the tread from sidewall to sidewall. The 55 is the aspect ratio — the sidewall height expressed as a percentage of the width. The R16 means it's a radial construction tyre designed to fit a 16-inch rim.
Understanding these three numbers is the foundation of any tyre size change — and getting them wrong can have serious consequences for your vehicle's handling, safety systems, and legal compliance.
mm
Ratio %
Construction
Diameter"
The aspect height = 205 × 0.55 = 112.75 mm
This is the sidewall height — the key measurement for diameter matching
Comparing Tyre Sizes Side by Side
When you go up a rim size, you must reduce the aspect ratio to keep the overall diameter the same — this is the golden rule of tyre size changes.
Moving from 16" to 17" rims? Reduce the aspect ratio to maintain the same overall diameter and keep your speedometer accurate.
Why Diameter Is the Critical Number
Your vehicle's speedometer, traction control, ABS, and gear ratios are all calibrated to the rolling circumference of your original tyres — which is directly determined by the outside diameter.
If you fit tyres with a significantly different diameter, your speedometer will display incorrect readings. A larger diameter means your speedo will read lower than your actual speed — potentially putting you over the speed limit without knowing it.
Beyond the speedo, incorrect diameter affects your traction control and ABS systems, which rely on wheel speed sensors. These systems can behave unpredictably if the tyre diameter is wrong.
The rule of thumb: keep the new tyre diameter within 1% of the original. Less than 0.5% variance is ideal. Our team can calculate this for you instantly — call 09 580 2403.
How to Calculate Overall Tyre Diameter
Four simple steps to work out the standing height of any tyre size — so you can compare old and new sizes before you buy.
Multiply Width by Aspect Ratio
Multiply the tyre width (mm) by the aspect ratio (as a decimal) to get the sidewall height in millimetres.
Convert mm to Inches
Divide the sidewall height by 25.4 to convert from millimetres to inches (there are 25.4 mm per inch).
Double the Sidewall Height
Multiply by 2 because there is a sidewall on both the top and bottom of the tyre (above and below the rim).
Add the Rim Diameter
Add the rim diameter in inches to get the total outside diameter — the standing height of the tyre.
Worked Example: 205/55R16
Important Safety Notice
Do not fit a different tyre size without first consulting an expert. Incorrect tyre sizing can lead to tyre failure, inaccurate speedometer readings, compromised traction control and ABS performance, and potential legal issues with your vehicle's WOF compliance. Call our team on 09 580 2403 before purchasing — we'll confirm the right size for your car in minutes, at no charge.
Found Your Size? We'll Fit Them at Your Door
Once you've confirmed the right tyre size for your vehicle, let Hyper Drive handle the rest. Our mobile tyre fitting service brings a fully equipped van to your home or workplace — no garage visit required.
- Order online, choose mobile fitting at checkout
- Professional fit and balance at your home or work
- Available across Auckland and major NZ cities
- From just $19.99 per tyre fitted and balanced
Tyre Size FAQs
Yes, in many cases — but there are limits. The tyre must fit within the wheel arch without rubbing on the bodywork or suspension components, and the rim width must be within the tyre manufacturer's recommended range. Wider tyres can improve grip but may also affect fuel economy and steering feel. Call us on 09 580 2403 for fitment advice specific to your vehicle.
As a general rule, the new tyre's overall diameter should be within 1% of the original. Less than 0.5% variance is ideal. Beyond 3% and you risk noticeable speedometer error, WOF issues, and potential interference with your vehicle's safety systems. Our team can calculate the exact variance for any size combination.
If you increase your rim diameter (e.g. from 16" to 17"), you must reduce the tyre's aspect ratio to maintain the same overall diameter. For example, 205/55R16 (standing height 24.88") is closely matched by 215/50R17 (standing height 24.96") — a difference of only 0.3%. This is the "plus sizing" concept used widely in the aftermarket wheel industry.
Potentially yes. NZTA regulations require that tyres are of an appropriate size and type for the vehicle. Significant size deviations — particularly those that cause tyre-to-bodywork contact or affect speedometer accuracy by more than a few percent — can result in a WOF failure. Always confirm with an expert before changing sizes.
No — you should never mix different tyre sizes on the same axle. Doing so creates uneven handling, puts stress on your differential, and can trigger ABS and traction control faults. All four tyres should ideally be the same size, brand, and model. At minimum, both tyres on the same axle must be identical in size.
