Short answer: take the rim width in inches, multiply by 25.4, add about 25mm, and round to the nearest tyre size. A 9 inch rim comes out at roughly 254mm, so a 245 or 255 is the right tyre. You have perhaps 20mm of slack either side of that before the tyre is stretched or ballooning, and the wheel, not the tyre, is what gets damaged when you get it wrong.
Nearly every order where someone asks us to double check their tyre sizes comes down to this one number. The wheel is right, the offset is right, and then there is a 225 going on a 9 inch rim because that is what was on the old 8 inch wheels and nobody thought about it.
It will fit. It will hold air. It is just not what the tyre was designed to do.
The quick rule
Rim width in inches, times 25.4, plus 25mm. That gives you the ideal section width in millimetres, which is the first number on the tyre.
So an 8.5 inch rim is 216mm of rim, plus 25, equals 241mm. A 235 or a 245 is spot on.
The reason for the extra 25mm is that a tyre is meant to bulge very slightly proud of the rim flange. That small amount of shoulder standing out past the metal is what takes the first hit off a kerb instead of your wheel lip.
Tyre width by rim width
| Rim width | Narrowest sensible | Ideal | Widest sensible |
|---|---|---|---|
| 7.0" | 195 | 205 – 215 | 225 |
| 7.5" | 205 | 215 – 225 | 235 |
| 8.0" | 215 | 225 – 235 | 245 |
| 8.5" | 225 | 235 – 245 | 255 |
| 9.0" | 235 | 245 – 255 | 265 |
| 9.5" | 245 | 255 – 265 | 275 |
| 10.0" | 255 | 265 – 275 | 285 |
| 10.5" | 265 | 275 – 285 | 295 |
Narrowest and widest sensible are the ends of the range a tyre manufacturer will normally approve for that rim. Going outside them is where the trouble starts.
One thing to note on the second number. If you are going wider to fill an arch, the profile usually has to come down to keep the rolling radius the same, or your speedo reads wrong and the car sits higher. A 245/35 and a 255/35 are not the same overall diameter.
What stretch actually costs you
Stretch is a narrow tyre pulled onto a wide rim, so the sidewall leans outward instead of standing up, and the tread sits flat or slightly domed. It is a look, and it is popular, so here is the honest list of what it costs rather than a lecture.
- Your wheel lip is now the outermost point. No shoulder standing proud means every kerb, every pothole edge and every speed bump lands on the rim. This is the single biggest cause of kerbed wheels on stretched setups.
- The bead is working harder than designed. A sidewall pulled outward is trying to lift the bead off its seat rather than press it on. Hard cornering on low pressure is when it lets go, and it lets go instantly rather than deflating.
- Less contact patch than the tyre's rating assumes. A domed tread rides on its centre, so you have less rubber on the road than the number on the sidewall suggests, and more wear down the middle.
- You need more pressure, not less. People run stretch soft to make it look better. That is the combination most likely to unseat a bead.
- Some fitters will refuse. Entirely their call, and a reasonable one if the combination falls outside what the tyre maker approves.
Mild stretch, one size down from ideal, is common and most fitters will do it without comment. Two or three sizes down on a low pressure is a different thing.
What a tyre that is too wide does
Less talked about, equally real. Too much tyre for the rim and the sidewalls pinch inward at the bead and balloon out in the middle.
You get vague steering just off centre, because the car has to roll the tyre over on its own sidewall before anything happens at the contact patch. It wears the outer shoulders. And the extra width has to go somewhere, which usually means fouling the inner arch liner on full lock or catching the strut.
It also raises the risk of the tyre rubbing at full droop over a bump when loaded, which you will not find on the driveway, only on a bad road with four people in the car.
Is stretched tyre fitment legal here?
There is no regulation in the UK or Ireland that names tyre stretch and bans it. That is the straight answer, and it is why you see so much of it.
What does exist is a general requirement that a tyre is of a suitable size and type for the wheel and is correctly fitted. An MOT tester who considers the combination unsuitable can fail it on that basis, and it is a judgement call rather than a measurement, so two testers can reach different views on the same car.
The bigger practical risk is insurance. Wheels and tyres outside manufacturer specification are a modification, and an undeclared modification gives an insurer a straightforward reason to query a claim. We went through that in do alloy wheels affect your car insurance, and the wider MOT and speedo side is in are bigger alloy wheels road legal.
Sidewall is what protects the wheel
Worth connecting this to the roads you actually drive on. The reason a stretched setup on a 20 inch wheel suffers here is the same reason a 35 profile suffers at all: there is barely any sidewall between the pothole and the rim, and stretch removes what little was standing proud.
If you are picking a size for Irish and UK roads rather than for a show, give yourself tyre. The figures for every common size are in why your alloy wheels keep buckling.
Running different widths front and rear is its own decision, covered in should you run wider rears.
Common widths in stock
- 20x9" Golf R Estoril Style, Gloss Black — 245 or 255 section — from £160
- 20x9" TTRS Rotor Style, Satin Gunmetal — 245 or 255 section — from £160
- 22x9.5" Autobiography Turbine Style, Black Machined — 255 or 265 section — from £215
- 23x10" Defender Urban Style, Gloss Black — 265 or 275 section — from £250
Kerbed one already? Every wheel is sold singly, see the single wheel replacement range.
Questions we get asked
What tyre width fits a 9 inch rim?
A 245 or 255 section is ideal. You can run a 235 at the narrow end or a 265 at the wide end and stay within what most tyre manufacturers approve, but outside that you are into stretch or ballooning.
How do I work out the right tyre width for my rim?
Multiply the rim width in inches by 25.4 to get millimetres, then add about 25mm. An 8.5 inch rim gives 216 plus 25, so 241mm, meaning a 235 or 245 tyre.
Is a stretched tyre illegal in the UK?
No regulation names stretch specifically. However tyres must be of a suitable size and type for the wheel and correctly fitted, so an MOT tester can fail a combination they judge unsuitable. Undeclared non-standard wheels and tyres also give an insurer grounds to query a claim.
Does a stretched tyre damage your alloy wheels?
It makes damage much more likely. A correctly sized tyre bulges slightly proud of the rim flange and takes the first impact from kerbs and potholes. Stretch removes that shoulder, so the rim lip becomes the outermost point and gets hit directly.
What happens if a tyre is too wide for the rim?
The sidewalls pinch in at the bead and balloon in the middle, giving vague steering just off centre and heavier wear on the outer shoulders. The extra width also tends to foul the arch liner on full lock or catch the strut.
Should I run lower pressures on stretched tyres?
No. A stretched sidewall is already pulling the bead away from its seat, and low pressure is the condition most likely to let it unseat. If anything a stretched setup wants pressure at the upper end of the recommendation.
Want your sizes checked before you order? Send the make, model and year, plus the wheel size you are looking at, to sales@omiwheels.com or WhatsApp +44 7753 828059 and we will give you the front and rear tyre sizes that keep the rolling radius right. We ship from Northern Ireland, so there are no customs charges or import VAT into Ireland or mainland Europe.