Here is a pattern we see constantly across Glasgow. A driver types their registration into a lookup site, gets a size, orders or books on it, and then the van arrives and the rubber on the car says something different. The question underneath it is always the same: how do I know what tyres fit my car?
So let us set the ground rules early. There are three sources of truth for your tyre size: the tyre sidewall, the placard in the driver’s door shut, and the handbook. The sidewall tells you what is fitted right now. The placard tells you what should be fitted. A registration lookup tells you what the car left the factory with, which is usually, but not always, the same thing.
This page teaches you to read all three, in plain terms, with every code explained. On our own jobs the wheel is checked on arrival before anything is touched, and by the end of this page you will be able to do the same check yourself.
The Short Answer: What Tyres Fit My Car
Your correct tyre size is on the vehicle placard in the driver’s door shut, the fuel filler flap or the handbook. The size fitted now is on the tyre sidewall, written as a code such as 205/55 R16 91V. A registration lookup is a starting point, not proof.
When all three agree, you are done. When they disagree, the placard is the manufacturer’s answer, the sidewall is today’s reality, and the difference between them is worth understanding before you spend money. The rest of this page shows you how to read each one.
Where to Find Your Tyre Size
The three sources answer three different questions, so check them in this order.
On the Tyre Sidewall
This is what is fitted right now, which is not always what should be. The size code runs across the widest part of the tyre.
Practical tips from doing this daily: if the numbers face inwards, turn the steering to full lock so the sidewall swings out. Wipe the rubber clean, because road film hides digits. And check all four wheels, not one, because a staggered set or a previous one-at-a-time replacement can make them differ corner to corner.
On the Vehicle Placard
The placard is normally in the driver’s door shut, on the fuel filler flap or in the handbook. It gives the original tyre size, sometimes front and rear separately, and the recommended pressures.
This is the manufacturer’s answer, and it is the reference point whenever what is fitted and what should be fitted disagree. It does not wear out, it does not get replaced by a previous owner, and it takes thirty seconds to find.
By Registration Number
A lookup returns what the vehicle left the factory with. That is usually right, and occasionally wrong, for reasons covered further down. Treat it as a good way to start a quote, and confirm it against the sidewall or the placard before money is spent.
| The placard | The sidewall | |
| What it tells you | What should be fitted | What is fitted now |
| Can it be wrong? | No, it is the manufacturer’s spec | It shows past mistakes faithfully |
| When to trust it | As the final reference | For quoting what is on the car |
Reading the Sidewall: A Worked Example
We ask callers to read the code out in order, left to right, because every part answers a different question. Take 205/55 R16 91V, the most common shape of code in the UK. Our full breakdown of tyre sidewall markings goes through every character on the wall, not just the size.
205, the section width
The width of the tyre in millimetres, measured across its widest point. It is the first number on almost every car tyre sold in the UK, and the easiest to find.
55, the aspect ratio
The sidewall height as a percentage of the width, so 55 means the sidewall stands 55 per cent of 205mm tall. A lower number means a shorter, stiffer sidewall, and it usually travels together with a larger wheel.
R, the construction
R means radial construction, which almost every modern car tyre uses. This is the one letter you can safely skip past.
16, the rim diameter
The wheel diameter in inches. It must match the wheel exactly. Width and profile have some flexibility in combination; rim diameter has none.
91, the load index
A coded maximum weight per tyre. Load index 91 means 615kg. The code is not a percentage or a pressure, it is a weight, and every code maps to a set figure. For any code other than 91, check the figure for your exact code against a published load index table rather than estimating.
V, the speed rating
The letter is a coded maximum sustained speed. The figures worth knowing:
| Rating | Speed |
| H | 130mph |
| V | 149mph |
| W | 168mph |
| Y | 186mph |
The rating fitted must be equal to or higher than the placard figure. Fitting lower is a compliance and insurance problem, not a preference.
Now read your own tyre the same way, left to right, and you have decoded the whole size.
The Letters That Come After the Size
The size is only half the sidewall. The letters after it decide whether a tyre that is the right size is also the right tyre.
XL or Reinforced
XL or Reinforced means extra load: a casing built to carry more weight at higher pressure. If your car came with XL, replace with XL. The rating is part of the fitment, not an optional upgrade, and dropping it quietly under-builds the corner of the car it sits on.
Run-Flat Markings
Run-flats are marked differently by brand: RSC, ROF, RFT, DSST, ZP and SSR all mean the same thing. A car built for run-flats often has no spare wheel at all, which is your biggest clue.
Mixing a run-flat and a standard tyre changes how the car behaves in a deflation, so know what your car was built for before choosing. The full comparison lives in our run-flat versus standard guide.
Manufacturer Approval Marks
Some tyres are tuned versions built for one car maker: MO for Mercedes, AO or RO1 for Audi, a star marking for BMW, N0 to N6 for Porsche, VOL for Volvo, AMV for VW Group.
The same tyre model can exist with and without the mark, with different behaviour. If your car came wearing an approved version, the approved version is the safe default at replacement. Unusual and approval marked sizes are confirmed on the call before the van sets off.
The C and CP Markings on Vans and Motorhomes
A C after the size, such as 225/75 R16C, means a commercial rated casing built for heavier loads at higher pressures. CP is the motorhome and camper specific version. Passenger XL and commercial C are different systems and are not interchangeable, and fitting a car tyre where the plate calls for a commercial rating leaves the casing under-engineered.
Two more van markings worth decoding. A dual load index such as 104/102 gives single-wheel and twin-wheel capacity, and single-wheel vans use the higher figure. Ply ratings like 6PR and 8PR are equivalent-strength ratings, not a literal count of layers.
How Old Are Your Tyres? The DOT Date Code
The last four digits of the DOT code on the sidewall are the tyre’s birth date: two digits for the week, two for the year. 2523 means week 25 of 2023, and the 7-7 rule is the industry shorthand people usually meet next.
Age matters even with plenty of tread left, because rubber degrades whether or not it is used. The signs are cracking in the grooves, crazing across the sidewall, and flat-spotting on vehicles that stand for long periods. We see the pattern most on motorhomes pulled out of storage before a trip, tread nearly full, sidewalls telling a different story. If yours shows cracking, our guide to tyres cracked with age covers what to do.
When a Registration Lookup Gets It Wrong
A lookup is right most of the time, and wrong often enough that we confirm the size on every job before fitting. Three situations cause nearly all the disagreements.
Staggered Fitments
Some cars, particularly performance and rear wheel drive models, leave the factory with different front and rear sizes. A lookup that returns one size is only half the answer. Check each wheel rather than assuming, and remember a staggered set cannot be rotated front to back, so the axles wear on their own schedules.
Aftermarket Wheels and Plus Sizing
Change the wheels and the tyre size changes with them. Plus sizing means a larger rim with a lower profile tyre chosen so the rolling diameter stays close to standard, which is where the 3 per cent tyre rule comes in. A non-standard size can be legal and safe when the load and speed ratings and the rolling diameter are right, but it should be confirmed against the placard before you buy. The lookup, meanwhile, keeps returning the factory answer regardless of what is bolted on.
A Previous Owner Fitted the Wrong Size
Used cars regularly arrive wearing a size that was never on the placard, and the current owner is usually the last to know. We find it when the wheel is checked before work starts. The fix is simple: compare the sidewall against the placard, and correct the size at the next replacement rather than matching the error for another set’s lifetime.
Do All Four Tyres Have to Match?
The law sets one hard rule here: a car must not carry tyres of different structure, meaning radial and cross-ply, on the same axle. Since nearly everything modern is radial, most drivers never meet that rule in practice.
Above the legal line sits good practice. Mixing tread patterns or brands across an axle is poor practice even where it is not itself an offence, because the two sides grip and clear water differently, and it is one of the things that can leave a car pulling to one side after new tyres. Replacing across an axle is the sensible approach where budget allows, keeping each axle matched even when the front and rear pairs differ.
On a staggered car, matching means matching per axle rather than across the whole vehicle, because the front and rear sizes are meant to differ.
What Fitting the Wrong Tyre Costs You
Three consequences, stated without drama.
The MOT. The legal minimum is 1.6mm across the central three-quarters of the breadth of the tread and around the complete circumference, and a tyre below that, cut, bulged or with exposed cord is a major or dangerous defect and an automatic fail. The RAC recommend replacing at 3mm, because braking performance drops sharply below it, and checking your tyres are legally safe takes five minutes.
Insurance. A defective tyre can be treated as a breach of the policy condition to keep the vehicle roadworthy, which can reduce or void a claim. A wrong rating is the version of this that drivers fit to their own cars without knowing, and the GOV.UK guidance on tyre safety sets out the driver’s obligations.
Load and handling. An under-rated casing on a loaded van risks overheating and failure, which is why a van wearing a passenger tyre gets corrected on our jobs rather than matched. We do not guess; we diagnose the problem and provide the safest and most cost-effective solution.
A Real Fitment Check, Start to Finish
The shape of a job we run week in, week out. A caller gives a registration and asks for two front tyres. The office asks for a sidewall photo anyway, because that is the habit that catches problems early.
The photo comes back showing a size that does not match what the lookup returned. A quick question about the placard settles which one the car should be wearing, and the quote is built on the confirmed size rather than the assumed one.
On arrival, the fitter reads the wheel again before touching anything, because the photo showed one corner and cars have four. The tyres fitted that day were the right size for that car, which sounds unremarkable and is the entire point. The wrong size repeated politely is still the wrong size.
How We Check It on the Job
Every job runs the same sequence, whoever calls and whatever the vehicle.
Quick questions on the call confirm the tyre size, your location and the issue. On arrival, the wheel is physically inspected before anything is touched, sidewall, placard and condition together. Then the fitting itself: balancing on the van’s on-board balancer, torquing to manufacturer specification, and a full safety check with wheel torque verification before the van leaves.
That inspection step exists because paperwork and rubber disagree more often than anyone expects. Lookups return factory answers, previous owners make quiet mistakes, and the only thing that settles it is reading the actual wheel. We are 24/7 Mobile Tyre Services, family run for over 20 years with fully certified technicians, and that habit of checking is most of what those years have taught us.
Frequently Asked Questions
How do I know what tyres fit my car?
Check the vehicle placard in the driver’s door shut, the fuel filler flap or the handbook for the correct size, and read the sidewall code on each tyre for what is currently fitted. A registration lookup is a useful start, confirmed against one of those two.
Where do I find my tyre size?
On the tyre sidewall, printed as a code such as 205/55 R16 91V across the widest part of the tyre. Your car’s intended size is on the placard in the driver’s door shut, the fuel filler flap or the handbook. Check both, on all four wheels.
What do the numbers on a tyre mean?
Taking 205/55 R16 91V: 205 is the width in millimetres, 55 the sidewall height as a percentage of width, R radial construction, 16 the rim diameter in inches, 91 the load index worth 615kg, and V the speed rating of 149mph.
Can I find my tyre size by registration number?
Yes, as a starting point. A lookup returns the size the car left the factory with, which staggered fitments, aftermarket wheels and previous owners can all make wrong. Confirm against the sidewall or the placard before buying anything.
Do all four tyres have to be the same size?
Not always, because some cars run staggered fitments with wider rears. The rules that do apply: no mixing of radial and cross-ply on the same axle, ratings meeting the placard figure, and ideally matching tyres across each axle even when the axles differ.
What does XL mean on a tyre?
XL, or Reinforced, means extra load: a casing built to carry more weight at higher pressures. If your car came with XL tyres, replace like with like, because the rating is part of the correct fitment rather than an optional extra.
How do I know if my tyres are run-flats?
Look for a brand-specific marking on the sidewall: RSC, ROF, RFT, DSST, ZP or SSR. A car built for run-flats usually has no spare wheel. We check this on every job across Glasgow before fitting, because run-flat and standard casings should not be casually mixed.
What is the DOT code on a tyre?
A code on the sidewall whose last four digits give the week and year of manufacture, so 2523 means week 25 of 2023. It tells you a tyre’s age regardless of tread, which matters because rubber cracks and hardens with time as well as use.
Get Your Size Confirmed
You now know how to read the sidewall, so put it to work. Photograph the code on one tyre, send it on WhatsApp to 07955 533000 with your postcode, and the size is confirmed and one total given on the call before the van moves. Our guide to what we need to quote for tyres covers the rest of that two minute call.
We are based at Polmadie in Glasgow and work 24 hours a day, 7 days a week, in all weather, across the city, the Central Belt and wider Scotland, fitting budget through Landsail and Triangle, mid-range like Falken and Hankook, and premium including Michelin and Continental, with mid-range suiting most drivers.
