Will they fit? The ultimate wheel and tyre fitment guide

You’ve got your eye on a new set of wheels. Maybe wider. Maybe with a bit of dish. You’re picturing them tucked just right, tyres sitting flush with the arches. But here’s the million-pound question.

Will they fit?

This guide walks you through everything. How to measure, what affects what, and how to avoid the dreaded rub, poke, or speedo lies. Once you’ve got your numbers, drop them into our wheel & tyre fitment calculator to see exactly how your setup will behave.

Tyre size explained: what those numbers mean

Let’s take a tyre size like 225/40 R18. Here’s what each part tells you:

• 225 is the width of the tyre in millimetres

• 40 is the aspect ratio, meaning the sidewall is 40 percent as tall as the tyre is wide

• R18 means the tyre fits an 18-inch diameter wheel

How to measure:

• Width is measured sidewall to sidewall when inflated

• Aspect ratio is a percentage, not a physical number

• R18 is the diameter of the wheel at the tyre bead, not the full outside diameter

When upsizing or downsizing, match the overall rolling diameter as closely as possible to your original tyre to keep the speedometer accurate. The calculator will handle that for you.

Wheel width: the tyre’s home base

Wheel width is measured across the inside barrel, not the outer lips. A “9J” wheel is 9 inches wide where the tyre sits.

How to measure:

• Measure from bead seat to bead seat

• Don’t include the outer lips or you’ll overestimate by around an inch

The width of your wheel affects tyre stretch, handling, and how far the wheel sits in the arch.

Wheel diameter

A “17-inch” wheel measures 17 inches at the barrel where the tyre bead seats. It is not the full edge-to-edge size of the rim.

To check:

• Flip the wheel over and measure across the inside barrel

• Ignore the outer edge or lip

This matters because tyre size depends on bead seating diameter, not the wheel’s physical height.

Offset (ET): what it really means

Offset is the number stamped as ET on your wheel. It stands for "Einpresstiefe," which is German for “insertion depth.” It’s measured in millimetres and tells you where the mounting face sits in relation to the wheel’s centreline.

• ET0 means the mounting face is exactly in the middle of the wheel

• Positive offset (ET+): Mounting face is towards the outside of the wheel. The wheel tucks in more.

• Negative offset (ET–): Mounting face is deeper into the barrel. The wheel pokes out more.

Real-world example: ET30 explained

Let’s say you have a wheel that’s 8 inches wide (about 203 mm).

• The centreline of the wheel is at 101.5 mm

• If your wheel is ET30, the mounting face is 30 mm towards the outside from that centreline

So the face is at 131.5 mm from the back edge of the wheel. That means:

• The wheel tucks in 30 mm more than an ET0 wheel

• It sits more inward in the arch

• Useful for cars with tight fenders, or to avoid poke

Now compare: ET45 vs ET30

Say your car came with 7.5J wheels at ET45. You’re looking at 8.5J wheels at ET30.

What changes:

• You gain 1 inch (25 mm) in width

• Offset drops by 15 mm

• The outer edge of the wheel moves out by about 19 mm

• The inner edge comes in by about 6 mm

In real life:

• Your wheels will look more aggressive, filling the arch more

• You might get closer to rubbing on the inside, especially with wider tyres

• Could require rolled arches if your suspension compresses aggressively

This is why using a fitment calculator is key. It shows exactly how a change like ET45 to ET30 affects your poke, clearance, and stance, with numbers and a visual.

Simple takeaway:

• Higher offset (ET40, ET50) = wheels sit more tucked in

• Lower offset (ET20, ET15) = wheels sit further out

• Negative offset (ET0, ET-10) = wheels are very far out, often used in off-road or wide-arch builds

Small changes in offset have a big impact on looks and clearance. Even 5mm can be the difference between clean fitment and rubbing on every bump.

Tyre stretch and sidewall behaviour

A narrow tyre on a wide wheel will stretch. A wide tyre on a narrow wheel will bulge. A bit of stretch can look aggressive. Too much is dangerous. Likewise, too much bulge can lead to vague steering and unpredictable grip.

The calculator shows you how different tyre widths behave on various wheel sizes, so you can strike the right balance between function and style.

Rolling diameter and speedometer accuracy

Changing your tyre profile or wheel size changes the overall rolling diameter. That affects:

• Ride height

• Speedometer accuracy

Larger diameter means your speedo will under-read. Smaller means it will over-read. Stay within 2 or 3 percent of the factory size for best results. The calculator gives you the exact difference and speed error in real numbers.

Ride height and arch gap

Wheel and tyre changes can raise or lower your car, even without touching the suspension. Bigger tyres might lift the body and increase arch gap. Lower-profile tyres can drop the car visually and reduce that gap. Your choice of offset also affects how much poke or tuck you get.

Final checks before you buy

Before you spend a penny, make sure you have:

• Your current wheel width, diameter, and offset

• Your current tyre size

• The specs of the new setup you want

Now plug it all into our wheel and tyre fitment calculator. It will show:

• How far your wheels will move in or out

• Inner and outer clearance changes

• Ride height difference and arch gap

• Speedometer error

Fitment is everything. Get it wrong and you’re into arch rolling, rubbing, or ruined tyres. Get it right and your car looks better, drives better, and turns heads for the right reasons.