Choosing the right wheels and tyres is about much more than appearance. Whether you’re upgrading your Ford Fiesta ST, Focus ST, Focus RS, or another performance Ford, understanding wheel offset (ET), wheel sizes, and tyre markings is essential to achieving the perfect fitment, avoiding rubbing issues, and getting the best performance from your car.
In this guide, we’ll explain exactly what wheel offset means, what every number on your wheel and tyre represents, and how to choose the right setup for your build.
⸻
What Do the Numbers on a Wheel Mean?
A wheel specification typically looks like this:
18 x 8J ET45 5x108 CB63.4
At first glance, it can look confusing, but each part tells you something important about the wheel.
Wheel Diameter
18
The first number is the diameter of the wheel, measured in inches.
Examples include:
- 15-inch
- 16-inch
- 17-inch
- 18-inch
- 19-inch
The tyre must match the wheel diameter. An 18-inch wheel requires an 18-inch tyre.
⸻
Wheel Width
8J
The number refers to the wheel width, measured between the tyre bead seats.
An 8J wheel is 8 inches wide.
Generally speaking:
- 7J = Narrower wheel
- 8J = Common performance size
- 8.5J = Wider stance
- 9J+ = Popular for track and aggressive fitments
The letter J simply refers to the wheel’s bead profile and is standard on almost every passenger car.
⸻
What Does ET Mean?
One of the most important measurements when choosing aftermarket wheels is the ET.
Example:
ET45
ET is short for the German word Einpresstiefe, which translates to offset.
Wheel offset is the distance, measured in millimetres, between the wheel’s mounting face and its centre line.
Think of it as where the wheel sits relative to the hub.
High Offset (ET50+)
A higher ET number means:
- Wheel sits further inside the wheel arch
- More suspension clearance
- Less wheel poke
Example:
ET50
The wheel sits further into the car.
⸻
Low Offset (ET35)
A lower ET number means:
- Wheel sits further out
- Wider, more aggressive stance
- More chance of rubbing arches
Example:
Changing from ET50 to ET35 moves the wheel 15mm further outward.
It doesn’t sound like much, but visually it’s a huge difference.
⸻
Why Offset Matters
Choosing the wrong offset can cause:
- Tyres rubbing the suspension
- Wheels fouling brake calipers
- Contact with wheel arches
- Increased steering kickback
- Premature wheel bearing wear
- Poor handling
Always choose an offset that’s appropriate for your vehicle and wheel width.
⸻
What Does 5x108 Mean?
This is known as the PCD (Pitch Circle Diameter).
Example:
5x108
This means:
- 5 wheel bolts
- Bolts positioned on a 108mm circle
Many modern Ford performance models use 5x108, including:
- Ford Fiesta ST Mk8
- Ford Focus ST Mk3
- Ford Focus RS Mk2
- Ford Focus RS Mk3
- Mondeo ST
- Kuga ST-Line
Older Fiesta ST150 models use a different bolt pattern (4x108), so always check compatibility before ordering.
⸻
What is Centre Bore?
Example:
CB63.4
The centre bore is the diameter of the hole in the middle of the wheel.
Ideally:
- Wheel centre bore = Hub size
If the wheel has a larger centre bore, spigot rings are used to ensure the wheel sits perfectly central.
⸻
Understanding Tyre Sizes
A tyre might be marked:
225/40 R18 92Y
Each number has a specific meaning.
⸻
225 – Tyre Width
Measured in millimetres.
A 225 tyre is approximately 225mm wide from sidewall to sidewall.
Common widths include:
- 195
- 205
- 215
- 225
- 235
- 245
- 255
Wider tyres generally provide more grip but may require wider wheels.
⸻
40 – Aspect Ratio
This is one of the most misunderstood numbers.
It is not the sidewall height in millimetres.
Instead, it’s a percentage of the tyre width.
For example:
225 × 40%
= 90mm sidewall height
Lower aspect ratios provide:
- Sharper steering
- Better cornering
- Sportier appearance
Higher aspect ratios provide:
- More comfort
- Better ride quality
- Improved pothole protection
⸻
R – Radial Construction
Nearly every modern road tyre is radial construction.
That’s what the R represents.
⸻
18 – Wheel Diameter
The tyre fits an 18-inch wheel.
Simple!
⸻
92 – Load Index
The load index tells you the maximum weight each tyre can safely carry.
Examples:
|
Load Index |
Maximum Load |
|
88 |
560kg |
|
91 |
615kg |
|
92 |
630kg |
|
95 |
690kg |
|
98 |
750kg |
Never fit tyres with a lower load rating than your manufacturer recommends.
⸻
Y – Speed Rating
The final letter indicates the tyre’s maximum approved speed.
Common ratings include:
|
Rating |
Maximum Speed |
|
H |
130mph (210km/h) |
|
V |
149mph (240km/h) |
|
W |
168mph (270km/h) |
|
Y |
186mph (300km/h) |
Although most cars will never reach these speeds, manufacturers specify them for safety and performance reasons.
⸻
Example Explained
Let’s decode this complete setup:
Wheel
18 x 8J ET45 5x108 CB63.4
Tyre
235/40 R18 95Y
This tells us:
- 18-inch wheel
- 8-inch wide
- ET45 offset
- 5x108 bolt pattern
- 63.4mm centre bore
- 235mm tyre width
- 94mm sidewall height
- Fits an 18-inch wheel
- Load capacity of 690kg per tyre
- Speed rated to 186mph
⸻
How Wheel Width and Offset Work Together
Offset doesn’t work on its own—it works alongside wheel width.
For example:
18x8 ET50
A conservative fitment that sits well inside the arches.
18x8 ET35
Exactly the same width but positioned 15mm further outward.
18x9 ET35
A wider wheel that extends even further towards both the suspension and the wheel arch.
That’s why simply asking “Will ET35 fit?” isn’t enough. The wheel width also needs to be considered.
⸻
Choosing the Right Fitment for Your Ford
Whether you’re fitting new alloys to a Fiesta ST150, Focus ST, Focus RS or another Ford performance model, always check:
- Wheel diameter
- Wheel width
- Offset (ET)
- PCD
- Centre bore
- Tyre size
- Brake clearance
- Suspension clearance
- Arch clearance
The right combination improves handling, appearance and safety, while the wrong setup can lead to rubbing, vibration and expensive repairs.
⸻
Final Thoughts
Understanding wheel and tyre specifications makes buying aftermarket wheels much easier. Once you know what each number means, you’ll be able to choose the perfect setup with confidence.
At Retro Fast Fords, we stock a wide range of performance wheels, tyres, suspension upgrades and accessories for Ford ST and RS models. Whether you’re building a subtle road car or an aggressive track-inspired project, choosing the correct wheel and tyre combination is the foundation of a great-looking and great-driving car.
Browse our range today and take the next step in your Ford build.
