Different Size, Right Fit: Rolling Diameter Explained

A temporary spare can look smaller and yet still be the right match. An easy-to-follow guide to explain.

Different Size, Same Journey

A temporary spare can seem smaller and yet still be the right match for your car. For example, your car has 19” wheels from the factory, but the Road Hero you receive is a 17” wheel.

What’s going on? It’s the wrong size, right?

What matters is the rolling circumference, let’s explain why this is important. When matching a spare wheel to a vehicle, there should only be a small tolerance difference in rolling circumference. This avoids your car’s systems becoming confused.

Wheel speed data is used by key systems including ABS, traction control, stability control, adaptive cruise control, engine and gearbox management, and even navigation. Because these functions depend on receiving accurate wheel speed data, having matching wheel sizes is critical to vehicle safety.

It may seem obvious, but your vehicle is programmed for all the wheels to be of a similar size. Disrupt that balance by fitting a wrong-sized wheel, and you could end up stranded, damaging the car, or potentially even having an accident.

Before explaining further, please note our in-house engineers design the fitment data behind our registration lookup so you don’t have to worry.

Road Hero gives you one spare wheel option, the correct option for your car.

So how can a different size wheel, be the right size?

This may sound illogical at first, but the issue is really the wording. The term ‘wheel’ that we’ve been referring to is really a combination of a wheel and a tyre together, and that’s the key part. The tyre. A spare can be different in size, as long as the height of the tyre makes up the difference.

Wheel size is only part of what makes up the total rolling circumference; the tyre makes up the rest.

This concept can be difficult to understand, so let me give a simple example: If we take a spare wheel, say a 17” wheel which is fitted with a tyre that has a 2-inch-high side wall:

2” tyre + 17” wheel+ 2” tyre = 21”

That could be matched to a vehicle that has 19” wheels with a 1” tyre sidewall.

1” tyre + 19” wheel + 1” tyre = 21”

Here’s a visual representation of that example:

Why is this so often misunderstood?

One of the reasons this is so often misunderstood or overlooked is the blend of using both metric and imperial measurements for tyres, it’s a legacy that hangs over the automotive tyre industry.

If you’ve ever looked at a tyre sidewall and thought it just looked like a jumble of numbers, you’re not alone. A size like 225/40 R18 mixes millimetres and inches in the same label. Why?

It’s down to history and standardisation. When the car industry was taking shape in the early 20th century, wheel diameters were first measured in inches, especially in the UK and US. By the time tyres became more advanced and manufacturers started quoting widths and sidewall ratios, much of the world had shifted to the metric system. The compromise? Keep rim diameters in inches for compatibility but use millimetres for tyre width and percentages for the sidewall.

Over the years, this hybrid stuck because switching everything to one system would create confusion, safety risks, and compatibility issues worldwide. In fact, Michelin once tried to launch a fully metric tyre system in the 1970s, but it failed because it couldn’t easily interchange with the existing inch-based wheels.

So today, tyre and wheel sizes remain a blend:

Wheel diameters → inches (e.g. 17”, 18”, 19”)

Tyre widths → millimetres (e.g. 205, 245, 305)

Sidewall height → a percentage of the width (e.g. 55 means 55% of 205 mm)

It looks strange at first glance, but this mix has become the global standard.

This mismatched system makes it difficult to calculate the overall wheel and tyre size, and therefore it’s just not something that is commonly referred to.

If that now makes sense, you understand that the wheels don’t have to be the same size if the overall wheel and tyre size matches, but you could be left wondering – why am I given a different size spare at all?

A practical example: Volkswagen Golf VIII

Having a spare wheel that’s a different size from your car’s normal wheels can be a confusing concept, so here’s a practical example to help explain:

Let’s take a 2.0 TDI 2020 VW Golf, from the factory this can come on an array of different sized wheels, from 15” through to 18”.

These wheels can come fitted with the following tyre sizes:

• 195/65R15 – Diameter 631mm

• 205/55R16 – Diameter 628mm

• 225/45R17 – Diameter 631mm

• 225/40R18 – Diameter 634mm

Different wheel sizes, different sidewall profiles, yet all with a similar diameter (and therefore rolling circumference) across the whole range of factory options.

TL;DR – Common questions, quick answers

Will a different rim size harm my car? The spare is specified to work within a tight rolling circumference window and for temporary use only. Obey the speed and distance guidance on the tyre and wheel label.

Do I need to calculate anything? No. Enter your registration, and we match the fitment to your exact model and braking setup behind the scenes.

Why not just buy any small wheel? Clearance, load ratings and mounting pattern matter. A kit exists so you do not have to experiment at the roadside.