I went to Trafico in Seville today, and they confirmed that if you have had your car licence (carnet B) for more than 3 years you are automatically allowed to ride a motorbike up to 125cc. I had read that one has to take a specific theory test, but today was very definitively advised that this is only necessary for >125cc, that for <125cc you do not need to do an exam and that your A1 licence is automatic after 3 years (as per real decreto 1598/2004, paragraph c).
To get a category A licence (they do not have any information about the proposed A2 licence -125-500cc- as it doesn’t come into force until December 8, 2009) when you already have a car licence you have to take a motorcycle specific theoretical test which is only available in Spanish (20 questions, 2 wrong permitted), and do 2 practical tests - 1 in a compound, 1 on the road - on a bike which is at least 220cc. You have to use a bike from a driving school because it has to be insured to conduct tests on.
If you don’t have a car licence you also have to do the common theory test, this is available in English, and doesn’t seem to change the price.
It’s 85? for the test, all parts included (though you’d have to pay the driving school for use of the bike).
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