The Strange World of Bike Measurements Explained

When talking about motorbikes we routinely bandy about units of measurement – brake horsepower, tyre sizes, tank capacity, fuel economy and so on, using units such as bhp, 120/70 17, 15 litres, and miles per gallon – but have you ever stopped to think about what they actually mean, and how bizarre some of them actually are?

Take tyre sizes for example: that ‘120/80 17’ refers to both metric AND imperial units, millimetres AND inches – but why?

Or fuel consumption: we routinely, still, talk about fuel capacity in litres, but fuel consumption in miles per gallon while our European cousins measure it, not as you might expect, in kilometres per litre but in litres per 100 kilometres. And don’t get us started on the Americans, who do use ‘mpg’ but whose gallons are different than ours…

So, we thought it’d be a bit of fun – and hopefully useful and educational, too – to look at some of the weirdest and most wonderful, and to explain, at least a little bit, what we’re actually talking about.

Tyre confusion? Blame the Americans…

A typical motorcycle front tyre dimensions these days is 120/70 ZR 17 – but what does it all mean? In simple terms, ‘120’ is the tyre width, in millimetres, ‘70’ is its aspect ratio, or profile (or height) as a percentage of its width. So, in this case, its profile is 70 per cent of its width (therefore, the lower the percentage, the lower the profile of the tyre). ‘ZR’ is its speed rating, which we won’t go into here. And 17 is the wheel’s diameter, in, er inches.

Why inches? Well, historically, pre-1970s, all tyres were measured purely in imperial units, with a typical front tyre from back then being something like 3.00 x 18, both in inches (tyres being narrower and larger diameter back then). Classic bike tyres are still labelled in the same way.

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