Free tool · no sign-up
Will that size actually fit?
Put your current size in one box and the size you are thinking about in the other. We will draw both to scale, work out the rolling-diameter change, and tell you exactly how far out your speedometer will be — plus the things that catch people out.
Ideal match — effectively the same rolling diameter
+0.38%
The accepted window is ±3%
- Overall diameter
- 632 → 634 mm+2mm
- Ride height at the arch
- +1mm
- Tread width
- 205 → 225 mm+20mm
- Sidewall height
- 113 → 101 mm-12mm
- Revs per mile
- 811 → 808
What your speedometer will read
Your speedo is calibrated to the original rolling diameter. Change it and every reading shifts by the same percentage — including the one the camera van is measuring you against.
Speedo reads
30 mph
You’re actually doing
30.1
+0.1 mph
Speedo reads
50 mph
You’re actually doing
50.2
+0.2 mph
Speedo reads
60 mph
You’re actually doing
60.2
+0.2 mph
Speedo reads
70 mph
You’re actually doing
70.3
+0.3 mph
UK law (and EU Regulation 39) requires a speedometer never to under-read: it may show up to 10% + 4 km/h more than true speed, but never less. A taller tyre eats into that margin and can push the car into under-reading, which is the direction that gets you a ticket.
Things to check before you commit
- 20mm wider than original — check inner arch and suspension-strut clearance, especially at full steering lock, and confirm the rim width suits the tyre.
- Going up in rim size adds unsprung weight and usually a firmer ride. It also means new tyres are more expensive to replace each time.
Going up a wheel size?
These sizes keep the rolling diameter of your 205/55R16 91V within 3% on a bigger rim.
The numbers are only half of it
A size can be perfect on paper and still foul an arch liner on full lock, or need a different offset, or sit wrong on your rim width. Tell us your registration and what you are trying to achieve and we will check the whole picture — for nothing, before you order anything.
What changes when you change size
Rolling diameter is the number that matters
Everything downstream — speedometer, odometer, ABS, traction control, gearing, even your fuel economy readout — is referenced to how far the car travels per wheel revolution. Keep the overall diameter within 3% and none of it notices. Go outside that and all of it drifts together.
Wider is not automatically better
A wider tyre puts more rubber down and usually improves dry grip and looks. It also adds weight, costs more, is slightly worse on fuel, and — because it floats more readily on standing water — is often worse in heavy rain. There is a reason winter tyres go narrower, not wider.
Lower profile buys you steering and costs you rims
Less sidewall means less flex, so the steering feels sharper and the car turns in more crisply. It also means less rubber between a pothole and your alloy. The bigger the wheel, the less there is between a Stockport pothole and your rim.
Offset and centre bore are not in this calculator
The tyre size is only half of a fitment. The wheel’s offset decides where the tyre sits in the arch, and the centre bore decides whether it sits concentric on the hub at all. Those depend on the specific wheel, which is why we check them against your registration before ordering.
Load index and speed rating are not optional
Whatever else you change, the replacement must meet or exceed the load index and speed symbol your manufacturer specified. It is the one part of this that is a hard legal line rather than a judgement call.
Fitment questions
How much can I change my tyre size by?+
The accepted guideline is to stay within ±3% of the original rolling diameter. Inside that window your speedometer stays within legal tolerance and the ABS and stability systems keep an accurate wheel-speed reference. Beyond it, both start to drift.
What is plus sizing?+
Going up in rim diameter while reducing the tyre profile so the overall rolling diameter stays roughly the same. Plus one is one inch bigger on the rim, plus two is two inches. You gain sharper steering and appearance; you lose ride comfort and gain pothole risk.
Will bigger wheels affect my speedometer?+
Only if they change the rolling diameter. A taller overall tyre makes the speedo under-read — you are going faster than it says — while a shorter one makes it over-read. This calculator shows the exact figure at 30, 50, 60 and 70mph.
Is it illegal to fit a different tyre size?+
Not in itself, provided the tyres are suitable for the vehicle, all tyres on an axle match, the load index and speed rating meet the manufacturer’s requirement, and nothing fouls. But a size that puts your speedometer into under-reading, or that reduces the load rating, will cause you problems at MOT and with your insurer.
Do I need to tell my insurer about different wheels?+
Yes. Wheels and tyres that differ from the manufacturer specification are a modification, and most policies require you to declare them. A quick call costs nothing; an undeclared modification can void a claim entirely.
Can you check whether wheels will fit my car?+
Yes — that is the part a calculator cannot do. We check PCD, offset, centre bore and real-world clearance at full steering lock against your registration before anything is ordered. It costs nothing and it is the difference between wheels that fit and wheels that go back.