Rear Tire Sizes - measurements

[Caravan Monster]

Long story short, I have come to the end of my supply of weird sized old knobblies that were given to me by a collector of works mx bikes when he sold his collection (tires are removed for shipping). These tyres had kept me going for a few years. The tires from this selection that worked best on my '06 EC300 were 4.50" Metzlers, but the clearance between tire and silencer is very close. I have been offered some mitas EF-05 and mitas CO2 in 120millimetre width at a good price, but I am not sure if they will be too wide for my bike. Anyone know if they will fit ? I'm not sure if we are using metric old or new in the UK.
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Gas Gas Man wrote:

I must have been under a rock.
There is a new tire size measurement for knobbies.
The old way was knobby width.
The new way is carcass width.
Here's the conversion.

Front:
Metric Old ....................... Metric ......................... Inch
60/ 100---------------------- 90/ 80----------------- 2.50/ 2.75
70/ 100----------------------90/ 90-------------------2.75/ 3.00
80/ 100---------------------100/ 80-------------------3.00/ 3.25

Rear:
Metric Old.........................Metric................ ....Inch
80/ 100-----------------------80/ 90------------2.50/ 3.60
90/ 100---------------------110/ 80-------------3.60/ 4.10
100/ 100--------------------120/ 80------------ 4.00/ 4.10
110/ 100-------------------130/ 80--------------4.00/ 4.50
120/ 100-------------------140/ 80---------------5.00/ 5.10
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(mitas CO2 are hard compound, long wearing tires with large knobs, good in mud, not FIM, which I think are sold under another name outside the UK market)