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Tyre sizes and departing from OEM spec

I'm having a little trouble finding the truth behind an assertion I read recently, that any variation in diameter of >3% from the OEM spec of your car, is illegal. Can anybody help? Is this correct, or another of those 'every fule kno' things?

A quick back of the envelope calculation suggests a 3% change in diameter would give you just slightly under a 10% change in the rolling circumference, which would potentially increase your car speedo error beyond the 'nominal' +10%/-0% that manufacturers are believed to be allowed*

Might be just a case that somebody's done the calc. and figured out that's the max change you can have before your speedo is legally unfit for purpose ?

*I haven't had my morning coffee yet, so that calc could be rubbish btw.. :(
 
911's always been renowned for excellent all day long brakes, even when they had (good looking (preferably Fuchs)) 15" wheels.

Athough it's probably also the case that an average modern shopping car could probably lap faster than them.
 
I'm always arguing that any modern road car with properly maintained brakes (new fluid, properly bleeded, pads not worn) can do a full emergency brake from top speed at least once where it is the grip from the tires that is the limiting factor, not the brakes. Do a track day and not all will survive, though, but how many SUV's with 22" and overly large Brembo's does that?

Taking a motorbike on a trackday and riding it hard only tends to add a bit of wear on very edges of the tyre but the brakes and tyres are otherwise fine. I don't think I've ever faded the brakes or overheated the tyres on a motorbike (not even when racing), but I've driven plenty of cars where the brakes are struggling after making progress on windy roads. When driving home to Edinburgh from down south I used to often turn off the A7 at Langholm and take the roads over the hills via Eskdalemuir and the Talla/Megget reservoir roads and with quite a few of my previous cars I'd need to back off when the brakes started to be a problem. Having said that, it's not been a problem when I've driven any of our current cars up there as their brakes seem more up to the job.
 
A quick back of the envelope calculation suggests a 3% change in diameter would give you just slightly under a 10% change in the rolling circumference, which would potentially increase your car speedo error beyond the 'nominal' +10%/-0% that manufacturers are believed to be allowed*

Might be just a case that somebody's done the calc. and figured out that's the max change you can have before your speedo is legally unfit for purpose ?

*I haven't had my morning coffee yet, so that calc could be rubbish btw.. :(

I make it out that 3% increase in diameter gives 3% reduction in indicated speed (circumference is diameter * pi). I'm pretty sure about the +10/0 rule is correct, speedos should indicate a bit to much to fool us into thinking we are going faster than we are. The GPS shows a much more correct speed.

Anyway. Who's gonna check your tire size? MOT? The Police?
 
It might be something that came up if you were involved in a serious accident - that'd be my only real concern.
No, it just means you’re more likely to receive the dreaded brown envelope telling you that you were one mile per hour too fast as you went downhill and round the corner from a 60 limit into a 30.
 
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I make it out that 3% increase in diameter gives 3% reduction in indicated speed (circumference is diameter * pi). I'm pretty sure about the +10/0 rule is correct, speedos should indicate a bit to much to fool us into thinking we are going faster than we are. The GPS shows a much more correct speed.

Anyway. Who's gonna check your tire size? MOT? The Police?

You're gonna make me do the work.. :)

Lets say a (cart) wheel has a diameter of 1m. Its rolling circumference will be 3.14m. Stretch the spokes by 3%. the rolling circumference will increase to 1.03m x 3.14 = 3.23m.

The net change in circumference is 0.09m, which as a percentage of the original is (0.09/3.14) *100 .. which equals approx 3%..

OK scrap that theory / (Note to self - steer clear of sums in the forenoon..)

Carry on .. :)
 
As mentioned, the relationship between diameter and circumference is a linear one, so 3% change to one equates to a 3% change to the other. And speedometers can over-read by up to 10%, but aren’t permitted to under-read. So if you have an over-reading speedo, a larger rolling circumference will reduce the amount of over-read. The question then is, if your speedo over-reads by <3%, then a >3% change in circumference would cause it to under-read, is this a problem, legally-speaking? It’s in the construction and use regs, so am I, as the user, bound by them or is it just the manufacturer who has to comply? It’s moot in my case as the speedo is now pretty much spot on, but if I went to a bigger tyre still, it’d start to under read.
 
It's a problem if you're caught speeding as a result of under-reading speedo and it may be a problem if the MOT tester spots non-standard size tyres and believes the size is inappropriate.
 
But if you’re aware, and know not to exceed, say 27 indicated in a 30 limit, would you still be illegal? And how much discretion does an MOT tester have over an opinion that something is ‘inappropriate’?
 
As mentioned, the relationship between diameter and circumference is a linear one, so 3% change to one equates to a 3% change to the other. And speedometers can over-read by up to 10%, but aren’t permitted to under-read. So if you have an over-reading speedo, a larger rolling circumference will reduce the amount of over-read. The question then is, if your speedo over-reads by <3%, then a >3% change in circumference would cause it to under-read, is this a problem, legally-speaking? It’s in the construction and use regs, so am I, as the user, bound by them or is it just the manufacturer who has to comply? It’s moot in my case as the speedo is now pretty much spot on, but if I went to a bigger tyre still, it’d start to under read.

Way back in the day when BMWs were still semi cool - I acquired a 325i with wide but grossly undersized tyres: 40% profile instead of 50% from memory. The 'rubber band look'. Was delighted when I ventured onto a ...private road..and managed to get the thing to an indicated 150 of your older imperial units. 'Twas only later when the penny dropped and I did the sums that I realised the thing wasn't quite as quick as it initially appeared :)

I can't see the tyre sizing ever being an issue in practice unless you end up in the unfortunate position of facing a very large insurance claim (on either side), and some diligent claims investigator/adjuster raises the issue and uses that as an excuse for his/her company not to have to pay out. Even there, probably more likely to surface as an issue in incidents involving the younger under 25 boy or girl racer / 'modified car using' demographic than for us chaps of a certain age. Hard one to raise with an insurance company though, as the simple response to 'Can I fit a tyre which is not listed as a manufacturers approved fitment for the vehicle ?' will invariably be a lazy but firm No.

I am aware of a road fatality here where the investigators discovered two of the tyres had been fitted in the reverse direction to that indicated on the side-wall - a practice oft used by car drifting aficionados for reasons Im not exactly clear about - and that was used as evidence to try to support the case that the driver was (a) a bit of a speed-demon and (b) had a wilful and reckless disregard for the constructions and use regulations. It didn't affect the outcome of the court case (all dangerous / other driving charges eventually dropped) but didn't read well for the driver.

Not much help to you I know Sue - apologies.
 
Thanks Alan. I think in order to invalidate the insurance, there might need to be an argument that the non-standard figment was in some way a contributory or aggravating factor to the accident. So tyres fitted the wrong way round, to facilitate drifting, implies their grip is less effective that way and could therefore be argued to be a causal factor. Similarly, if you fit much wider tyres and aquaplane into a bus stop, that would probably be deemed a contributory factor. But tyres of the same width, and marginal additional rolling diameter are, as my insurers confirmed, not a material change.

I’m leaning to the view that what was said elsewhere about it being illegal was, to use the correct technical legal term, bollocks.
 
A quick back of the envelope calculation suggests a 3% change in diameter would give you just slightly under a 10% change in the rolling circumference, which would potentially increase your car speedo error beyond the 'nominal' +10%/-0% that manufacturers are believed to be allowed*

Circumference and diameter and linearly related c= pi * d so a 3% change in d leads to a 3% change in c. I think you maybe used the area calculation a = pi * (d/2)^2
 
A Swedish mag actually tested tires fitted the wrong way, both in direction of travel and inside to the outside. Very little difference in performance, one was even a tad better the wrong way! The major difference was in noise.
 
Circumference and diameter and linearly related c= pi * d so a 3% change in d leads to a 3% change in c. I think you maybe used the area calculation a = pi * (d/2)^2

You're seriously overestimating my current mathematical ability Sean - especially at that time of day :)

I did the quick calc on the differences between circumferences on a 1.0m vs 1.03m diameter wheel on my phone calculator, got a figure of '0.09' and decided that meant there was a 9% difference...

(I used to be in gainful employment as an aeronautical engineer would you believe. Some time ago. Obviously :) )
 
A Swedish mag actually tested tires fitted the wrong way, both in direction of travel and inside to the outside. Very little difference in performance, one was even a tad better the wrong way! The major difference was in noise.

I had a car where the tyre fitter put 2 of the tyres on the wrong way around. They were most disgruntled when I took it back and demanded that they sort it out.
 
Just had to buy a new Pirelli P7 for my S type - Tyre place used to hold them in stock, today he managed to find the only one in the South East area - sign of the times?
 


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