The gauge has two slots, one will measure up to 4.5mm and the other up to 6mm. This will determine which strap you need.    

 

Try the 4.5mm slot FIRST, and then the 6mm slot.  

The door edge must fit all the way into the slot without force.

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Be GENTLE and DO NOT FORCE the gauge on, this may damage the paintwork.

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If the door edge fits into the 4.5mm slot, you will need a Standard Strap.

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If you have to use the 6mm slot, you will need a Standard Large Strap.

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If there is a seal obstructing the gauge, you will need a Universal Strap.

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If it is too wide for the gauge, you will need a Universal Strap.

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Then, if there is NO SEAL at the door edge, you may need a Ribbed Universal Strap.