Condition and maintenance of tyres27.
(1) Save as provided in paragraphs (2), (3) and (4), a wheeled motor vehicle or trailer a wheel of which is fitted with a pneumatic tyre shall not be used on a road, if—
(a)the tyre is unsuitable having regard to the use to which the motor vehicle or trailer is being put or to the types of tyres fitted to its other wheels
The Road Vehicles (Construction and Use) Regulations 1986
The law, courts and the police, use specifications, not owner manuals.
Load ratings and speed ratings (otherwise called the 'service conditions' ) are the only applicable specifications, where it is suspected the tyres are 'unsuitable' regarding the use to which they are being put.
Using a 600kg load rated tyre on a 500kg load is perfectly acceptable in the eyes of the law, as is using a 130mph rated tyre when the maximum speed is 70mph. Or else otherwise we need to question the purpose of a load rating and ask if it is adequate to describe a particular characteristic of any tyre.
Tyre derating is a perfectly acceptable practice, in fact it is mentioned in the MOT notes and guidance issued by VOSA.
Originally Posted by VOSA guidance, class 5 and 7
Unless the vehicle is a ‘Restricted Speed Vehicle’ the tyres are required to be suitable for use up to the maximum prescribed speed limit of 70mph, ie ‘L’ speed rating. Tyres of the lower speed ratings of ‘J’ or ‘K’ however are acceptable for use at 70mph although the increase from the nominated speed rating imposes a reduction in the tyres’ carrying capacity
Note: This allowance is only applicable to the nominal service markings (Load Index/Speed Symbol).
This allows a tyre displaying a ‘J’ speed rating (suitable for a maximum of 62mph) to be used at the ‘L’ speed (suitable for a maximum of 70mph) at the penalty of reducing the tyres capacity by 7%.
In the case of a tyre displaying a ‘K’ speed rating (suitable for a maximum of 68mph) a reduction in capacity of 3% is imposed to allow use up to the ‘L’ speed.
e.g. 146/143K= 6000kg single/10900kg dual – at a maximum speed of 68mph
Less 3% = 5820kg single/10580kg dual – at a maximum speed of 70mph
This would allow a tyre displaying the speed rating ‘K’ to be used on a vehicle to which a maximum prescribed speed limit of 70mph applies, subject to it being suitable at the reduced capacity of 5820kg in single and 10580kg in dual formation, for the maximum permitted axle weight of the axle to which it is fitted (the GB maximum permitted weight as shown on the manufacturer’s plate).
Inspection of load and speed rating is not carried out on passenger cars at the MOT because it is generally assumed that the only tyres which will fit passenger car wheel rims of a particular size are tyres which can easily carry the weight of a typical passenger vehicle fitted with that rim size. I know that sounds unscientific, but remember that very little has changed since the day when the standard tyre profile was an 80% profile. In fact 80% is still considered to tbe the standard tyre profile.
However it is generally understood by tyre manufacturers that because they cannot produce one tyre suitable for each individual axle of each car ever made that the guideline specifications (load and speed) may be reconsidered depending on the use to which the tyre is being put, with no adverse affects.
In the eyes of the law, the owner of a vehicle needs to only make sure the tyres are suitable for the maximum load stated on the weight plate as well as accept they will not drive at a speed beyond the speed rating of that tyre. In so doing they are meeting the requirements of the cons and use regs.
I wouldn't recommend anybody should break the law, and in order not to do that, we should always pick tyres which have specifications which at least meet the demands of the application to which that they are being put.(using the weight plate of the car and considering the maximumm speed which the car can be driven in public ). For years, many people have used a rule which says "Load requirement + 20%" and that serves well for practically any vehicle
Being that the tyres have to be suitable for the maximum permitted weight, and assuming you have meticulously weighed your passengers and their luggage, you can consider the tyre which is suitable for the full load to also be suitable for the more typical lower loads too.
Try answering this simple question -
What is the maximum load a 98 W rated tyre can carry at it's maximum speed of 168 mph ?