Why cross plys and radials don't mix

Find out what could happen if you mix your radials with your cross ply tyres.

Radial tyre sizes explained

Radial tyre sizes are expressed as a series of numbers and letters. For example, the tyre code might read 185/65 R15 88 H. But what does it all mean?

• 185 is the cross section width
• 65 is the aspect ratio
• R is tyre construction
• 15 is the rim size
• 88 is the load index
• H is the speed rating

Let's start at the beginning. The '185' refers to the cross section width - the width of the tyre. It is given in normally given in millimetres and measures from the tyres inner sidewall to it's outer.

The second number '65' is the aspect ratio - or the profile of the tyre. This is the height of the sidewall given as a percentage of the section width. So in our example, it is 65% of 185mm. The bigger the aspect ratio, the bigger the sidewall.

The 'R' is easy - it stands for the tyre construction, which here is radial. It's followed by the diameter of the rim, measure in inches. So here, it's a 15-inch wheel.

What about the 88? That represents the load index. The load index is the maximum load in kilograms that the tyre can carry. So a load index of 88 means a maximum load of 560kg (see seperate chart).

The final piece of information is the speed rating. Here, our tyre is rated H, which means it's rated for a top speed of 130mph (see chart).

How to check your classic car's tyres

Watch as Fuzz Townshend shows you how to make sure your rubber is road legal.

Tyre date codes, what do they mean?

Tyre date codes were introduced in the 1980s. So if your tyre doesn't have a date code, then it's old and dangerous - the rubbers is past its best and is at risk of cracking and should be replaced.

In the 1980s, a system was introduced to let people know when the tyre was made. It's was a simple three-digit code where the first two digits specify the week of the year and the last number is the year itself. So, for example, 132 would be the 13th week of 1982.

This system was used in the 1990s, but this time had a small triangle inserted after the code to signify the decade.

From 2000 onward, the system was updated to a four digit code but uses a similar principle. The first two digits are the week and the last two are the year. A tyre that carries the date stamp 2112 then it was made in the 21st week of 2012.

Profile: Michelin

Back in 1899, brothers Edouard and Andre in 1899 ran a rubber plant. One day, a cyclist turned up at the factory with a pneumatic tyre in need of repair. It was glued to the rim, took three hours to repair, needed to be left over night, and failed the next day. Convinced they could do better, the Michelin brothers came up with their own design and Michelin was founded in May the same year.

By 1934, they'd come up with a design for a tyre which, if punctured, would run on a special foam lining (today we call it a run-flat tyre). But the big breakthrough came in 1946 when the company gave us the radial tyre.

As you might expect, the French tyre manufacturer worked closely with domestic car makers and the radial tyre (intially called the X-tyre) was developed for the front-wheel drive Citroen Traction Avant (Michelin had in fact bought Citroen in the 1930s). The radial was a run away success offering superior handling and fuel economy compared to the cross-ply. 

In 1975, Michelin had another big idea - the TRX ('Tension Repartie' or distributed tension), which proved itself in Formula One with with Renault and Prost and in the world rally championship on the Audi Quattro, Peugeot 205 Turbo 16 and Renault 5 Turbo.

Here, the tyre and the rim worked together to offer improved directional stability. it was fitted as original equipment to a number of cars, including BMWs, the Citroen CX 2400 GTI, Peugeot 604, Ferrari 308 GTS and GTB, Alpine A310 V6, Renault 5 Turbo and Peugeot 205 Turbo16.

Specialist Profile: Dunlop

Dunlop's story began in 1888 when founder, John Boyd Dunlop, wanted to come up with better tyres for his son's tricycle. The subsequent design (wheels wrapped in thin rubber sheets, glued together and inflated) was the blueprint for the first pneumatic tyre.

In 1890 Dunlop opened its first tyre plant in Dublin, Ireland, and by the start of the Second World War Dunlop was making brakes, wheels, golf and tennis balls. 

Plenty of classics left the factory wearing Dunlop rubber, not least the Jaguar E-type. Hardly surprising then that Fort Dunlop (the home of the original tyre factory and head office) was located in the Midlands just a few miles from Jaguar's Castle Bromwich site.

Specialist profile: Pirelli

For a company so well known for it's racing rubber, it's perhaps surprising that Pirelli initially specialised in scuba gear. But that was way back in 1872.

Pirelli's big idea came in 1974 with the creation of the wide radial tyre - a special request from the Lancia rally and race team who needed a tyre that could handle the insane power put out by the new Stratos (which was busy destorying the existing crop of radials within 10km).

The wider rubber boasted a reduced sidewall height like a slick but used a radial structure. In fact, it was so good it caught the eye of Porsche who promptly fitted it to the 911 Turbo.