How much change in profile is too much?

[DaveE87]

As per title, how much change in profile is too much? A quick Google finds conflicting information. Bridgestone advise within 3% of diameter. Protyre suggest only 1.5% up, or 2% down.

I have a Volvo C30 D5 with R design suspension, Midir wheels and 225 / 45 / 18 tyres. Hitting a pothole caused a slight buckle in one of the wheels so I'm considering getting a whole new set. If I went down to a 17 inch wheel, can I increase the profile to 55 or even more?

[GreenV8S]

It's important to keep the outer diameter of the tyre the same. If you get this wrong you will throw out the speed calibration (which can affect legality and insurance), potentially upset TC and ABS systems, potentially damage 4wd drive systems.

How much of that diameter comes from the rim versus the tyre and how wide the tyre is affects ride and handling so you want to be cautious about big changes, but it isn't going to damage anything and there's no absolute limit you could say is right or wrong.

[Pica-Pica]

Try here and compare
http://www.wheelcalc.com/
Going to 225/50 -R17 should be very close. What does your tyre label on the car suggest as a 17” alternative.
When you find a size that works out technically, ask your insurance about the change, they can be funny if you don’t tell them. They will almost certainly say OK (if technically OK), but they need to be kept informed.

Edited by Pica-Pica on Friday 2nd June 12:24

[Smint]

What alternative sizes are stated in the handlook or the tyre pressure label, hopefully there's an approved alternative but keep in mind clearances required for brake calipers if your model has larger brakes fitted.

Daughter's chap has had several Civic type R wheels buckled by pot holes recently, every one straightened out by a specialist.

[wyson]

Yes, just fit one of Volvo’s standard wheel and tyre sizes for the C30. Lower spec models would have come with smaller wheels and fatter tyres. Much less headache that way.

Edited by wyson on Friday 2nd June 14:08

[MustangGT]

I work on a maximum 2% change. Increasing the rolling circumference by 2% can 'remove' the speedo error in a lot of cases.

For your 225/45/18 I would say that the following would be acceptable in 17" (providing you can clear the brake calipers):

225/50/17 (99.56% of standard)
215/55/17 (101.3% of standard)

225/55/17 is almost 3% (102.97%) larger. This could cause your speedo to under-read.

If 16" wheels fit then the following:

225/55/16 (99.12% of standard)

Another thing to look at is what were standard size wheels/tyres on different spec of the car. Use one of those and the insurance company will probably not mind the change. They will need informing in any event of a change.

[DaveE87]

GreenV8S said:

It's important to keep the outer diameter of the tyre the same. If you get this wrong you will throw out the speed calibration.

Upon checking, the tyres are already slightly larger than spec 225 instead of 215. The speedo runs true (according to Waze) all the way up to about 70.

Pica-Pica said:

Try here and compare
http://www.wheelcalc.com/
Going to 225/50 -R17 should be very close. What does your tyre label on the car suggest as a 17” alternative.
When you find a size that works out technically, ask your insurance about the change.

I've been using something similar - willtheyfit.com which shows a visual mockup too.

Unfortunately the tyre label only references the donut wheel size.

I'm running non-standard wheels on my other car, however the insurance company weren't fussed when I told them. That's on a modified policy though if that makes a difference.

Smint said:

What alternative sizes are stated in the handlook or the tyre pressure label, hopefully there's an approved alternative but keep in mind clearances required for brake calipers if your model has larger brakes fitted.

Daughter's chap has had several Civic type R wheels buckled by pot holes recently, every one straightened out by a specialist.

As above, it just references the donut wheel.

This is just a commuter so everything is standard, but I've had to deal with big brake issues before.

I hadn't considered just getting a repair tbh. That's probably the cheapest fix. When I bought the car the previous owner had just fitted 4 new budgets, so I'm doing some man maths to try justify getting some premium tyres instead.

wyson said:

Yes, just fit one of Volvo’s standard wheel and tyre sizes for the C30. Lower spec models would have come with smaller wheels and fatter tyres. Much less headache that way.

That too is an option.

[DaveE87]

MustangGT said:

I work on a maximum 2% change. Increasing the rolling circumference by 2% can 'remove' the speedo error in a lot of cases.

For your 225/45/18 I would say that the following would be acceptable in 17" (providing you can clear the brake calipers):

225/50/17 (99.56% of standard)
215/55/17 (101.3% of standard)

225/55/17 is almost 3% (102.97%) larger. This could cause your speedo to under-read.

If 16" wheels fit then the following:

225/55/16 (99.12% of standard)

Another thing to look at is what were standard size wheels/tyres on different spec of the car. Use one of those and the insurance company will probably not mind the change. They will need informing in any event of a change.

2% does seem to be about the norm. My speedo runs true at legal speeds due to the slight difference from standard already on the car. I posted on a C30 group too and a few people are running all sorts. One was even on Ford Kuga 19's with fairly chunky tyres.

The standard brakes are tiny, would be fine with a 16".

I'd run it past the insurance, I imagine there won't be much of any issue if I'm sticking to the standard stud pattern.

[GreenV8S]

DaveE87 said:

Upon checking, the tyres are already slightly larger than spec 225 instead of 215. The speedo runs true (according to Waze) all the way up to about 70.

That's unusual. Speedos are allowed to read slightly high but only within a very narrow tolerance, and are not allowed to read low. So they usually read a few percent high as standard. I expect the non standard tyre you're using has used up all that tolerance.

[E-bmw]

GreenV8S said:

DaveE87 said:

Upon checking, the tyres are already slightly larger than spec 225 instead of 215. The speedo runs true (according to Waze) all the way up to about 70.

That's unusual. Speedos are allowed to read slightly high but only within a very narrow tolerance, and are not allowed to read low. So they usually read a few percent high as standard. I expect the non standard tyre you're using has used up all that tolerance.

That looks about right as my (very similar underneath) V40 & previous V50 both just read 2/3 mph high.