Why are tyres dual marked ?
Have you ever been confused or wondered why tyres are often marked with multiple sizes on them such as 500/600x19? Or you order a 550x18 tyre and what arrives is marked up 525/550x18 or 550/600x18 on the sidewall?
The reason is much simpler than you might think. It's what we can only call "Marketing Bullshit"! And unfortunately a generation or two have now been trained up expecting this.
In period there was no such thing as a "dual marked" tyre, as will be borne out from looking in original tyre catalogues, or knowing about the cars themselves. So what happened?
Basically the tyre suppliers decided to cut the choice of tyre sizes available, but wanted to keep the illusion of having exactly the same range, for fear of losing sales of what they cut back on to a competitor. So as an example, not so long ago Dunlop stopped making it's 550x18 race tyre but continued to sell it's 600x18 tyre, but with the mould now re engraved to read 550/600x18. But this is their 600x18 tyre which is now dual marked. It is not too complicated to work out what happened here. It is what happens when management and accountants and clever people decide to maximise profit, while hoodwinking the end user. I became aware of this when a friend bought a single 550x18 replacement tyre and found it so much bigger! In reality Dunlop do not make a 550x18 tyre, but they still list it.
In the earlier days of Blockley, Michelin made a dual marked "500/600x21" tyre, so if a customer asked a wholesaler like Vintage Tyre Supplies or Longstone Tyres etc for a 500x21, a 525x21, a 550x21 or 600x21, or mentioned some dual marked 550/600x21 or some other combination of sizes, they would get sold the very same singular tyre covering all those sizes marked on the sidewall 500/600x21, which was in fact a 600x21 tyre. These tyre sellers advertise "Call us for expert advice" (one of them even advertised that they are the "Ultimate Tyre Authority"!) and usually sell whatever makes them the most margin!
When Blockley made the 600x21 Blockley tyre (on the advice of Michelin who told me they would be withdrawing this size) it was suggested that I ought to dual mark the Blockley so as not to be disadvantaged in the market place by the "Classic tyre wholesaler experts", and reluctantly I did so, calling it "525/600x21". When Coker subsequently copied many of my Blockley tyres ("monkey see, monkey do") in their attempt to shut the Blockley project down, they chose to call their copy of my tyre 550/600x21.
The less tyre sizes you make, the less inventory is needed, and with all this double or more sizing giving the illusion of a wider range, the more profitable. And profitability is what it's all about, and it works even better by controlling the market as a few handful try to dominate the world. Blockley is the only company to have been started with a different ethos to all this, which is why we have come across so much pressure from those in charge of the Classic tyre market.
This same multisizing process works even better with inner tubes, where the range is now so multisize marked up that we had to make our own Blockley inner tubes, as we found the situation unacceptable, driven by profit and sometimes even "brand" rather than quality. But that is another story.
So when you see a multi size marked tyre, what size is it? A good rule of thumb is if a tyre has dual marking, the actual tyre it is the larger of the sizes quoted. So for example 550/600x18 is a straight 600x18 tyre out of the original 600x18 tyre mould. So as you can now appreciate, you cannot have 2 sizes of tyre in one tyre - or can you?
But with Blockley, and Blockley only, since we design and make all our tyres from scratch with brand new moulds we are starting from a clean sheet of paper, rather than those those hunting down worn out scrap to revitalise. So for example in the case of Blockley a 475/500x19 really is a genuine mix dimensionally of these 2 sizes. 450/500x18 similarly so. Most of our dual marked sizes are an amalgam of the sizes unlike any other brand, although in a few cases they are not. At Blockley we really abhor this dual marking business, but this is the way of the world and we try and do this to an absolute minimum. One of our latest tyre sizes produced is 440/450x21 which is a case in point, which we have had to do the dual marking on because that is what everyone asks us for. . . .