there are plenty of places to buy tires in guatemala city there is a place called FPK every biker knows that place located at 10 calle 4-38 zona 9 phone# (502) 23611923 and you can find tyres there, I went like two moths ago and bougth front and back tyre for my africa twin for like $150 both , also there is Moto Punto where you can find tyres too is located at 4 ave. A 13-24 zona 9 phone # (502) 23609522 Im pretty sure anyone can give you info how to find both places, and your 3rd choice would be Bavaria Motors s.a at 8a ave . 4-54 zona 4 phone # (502) 23313335 I know you probably wont believe that from bmw dealer, but sometime they have good prices !!!!! good luck.
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