Increasing tyre diameter - legal?

[SRLA]

Hi guys, I've been trying to find an answer to this for a while and can't seem to get a definitive answer. I'll have to change tyres soon and because of the rubbish roads around here I want to go for something with a bit more sidewall, currently running 225/35 and 255/30R19s front/back, respectively, and I'm thinking of going up to 225/40 and 255/35. This is about a 3.5-4% increase in wheel diameter, I've been trying to find out the legality of it and I'm finding info saying not to go up by more than 1.5-3% (varies with the source) but nothing legal and I can't see anything on the government MOT guidelines about this.

Now I'm not concerned with the change in speedometer reading because it reads a good 10% over as it is so even with a 4% larger wheel it'll still read over but by less so which is fine by me, what I don't want is to have the car fail at the next MOT because of this. Does anyone know a straight answer to this?

Thanks

[stevieturbo]

you need to maintain a similar rolling circumference to existing F/R tyres respectively, otherwise car systems will not be happy

Going too big may run into clearance/fouling issues.

If you want a softer ride, then you need smaller wheels, so you can really have a larger sidewall tyre.

Legally ?

if it is considered a "modification", you will likely need to tell the insurance.

Although people don't need to tell insurance when they fit cheap dangerous chinese ste tyres, so why you would when changing others is a bit of a mystery.

https://www.willtheyfit.com/

[SRLA]

Yes I'm changing both front and rear so the diameters still match between the axles. It's not a massive increase - about an inch total so only half of that between the wheel and arch, I think clearance will be fine I'm not lowering the springs and there's a decent amount of room for wheel travel. I really want to avoid changing the wheels if I possibly can, these look good and suit the car and going for smaller wheels then I'd have to worry about the brake calipers fitting, look for a staggered 18" set, I'd really like to avoid all that hassle, I just want something that will help deal with potholes and these ridiculous raised zebra crossings better (I'm also replacing the shock absorbers but keeping the same ride height).

As long as they don't fuss over it on the MOT I'm happy, not overly concerned about the insurance side of things (fk 'em!), it's as you say it's ridiculous to suggest my car would be more of an insurance risk with some new Michelin P4Ss with slighter higher sidewall than OEM than something out there with OEM size Chinese plastic tyres!

[E-bmw]

SRLA said:

As long as they don't fuss over it on the MOT I'm happy,

They won't even look at the sizes, never mind check them to oe spec.

[Scrump]

The winter tyres I have for one of my cars have a slightly taller sidewall (front 40 to 45 and rear 35 to 40). I am not convinced this has made a major difference to the ride quality. Choosing different brands of tyres has made a bigger difference.

[Dingu]

SRLA said:

As long as they don't fuss over it on the MOT I'm happy, not overly concerned about the insurance side of things (fk 'em!), it's as you say it's ridiculous to suggest my car would be more of an insurance risk with some new Michelin P4Ss with slighter higher sidewall than OEM than something out there with OEM size Chinese plastic tyres!

Don’t come whinging and asking for help if you run into an issue with your undeclared modification then.

[GreenV8S]

SRLA said:

it's ridiculous to suggest my car would be more of an insurance risk ...

There's a legal requirement in the UK that the speed never reads less than the actual speed. It also isn't allowed to read more than 10% higher than actual speed at a specific speed. This means you need to stay within a fairly small margin of error to stay legal.

Increasing the wheel size would make it read lower. It wouldn't need much of a change to make it read less than the actual speed.

You can debate how likely it is that you'd be found out and that anyone thought it was important enough to pursue, but you do need to be clear that if you stray outside the legal limit then your car becomes non-road-legal (and also probably uninsured) due to a defect that arguably impacted safety.

If you truly believed it made no difference to the insurance risk then of course you'd disclose it.

[anonymous-user]

The proper way to do this is to get the next size wheel down, which will probably have the same tyre width and the next profile size up to compensate. With many cars you can use the manufacturers software to tell the car what size wheels it has fitted so it knows what's going on.

Increasing the size of the wheel/tyre could well cause the tyre to foul the arch on full lock and rub. Easily ignored though!

[stevieturbo]

SRLA said:

Yes I'm changing both front and rear so the diameters still match between the axles. It's not a massive increase

You're making a 4% change at front, and almost 7% change at the rear.

As said, you need to ensure F/R rolling circumference is kept the same relative to OE fitment respectively.

4% and 7% are not the same. Although it is now a 3% discrepancy F/R

Do so at your own risk. Car systems may not be happy. I don't give a F about the speedo.

Edited by stevieturbo on Tuesday 3rd January 17:59

[Pica-Pica]

GreenV8S said:

SRLA said:

it's ridiculous to suggest my car would be more of an insurance risk ...

There's a legal requirement in the UK that the speed never reads less than the actual speed. It also isn't allowed to read more than 10% higher than actual speed at a specific speed. This means you need to stay within a fairly small margin of error to stay legal.

Minus 0 to plus 10% +4kph over read. UN-ECE regulation 39