I would say, "ON average" and not "IN average". On average is a set adverbial phrase. If you want to use in average, you would have to write a different type of sentence construction. For example, In average cars there are 4 cylinder motors. In this sentence, average is functioning as an adjective and describes cars, the preposition is not tied to average as it is in the adverbial phrase on average. I hope this helps. Quero esclarecer dúvidas e não aumentá-as. Um forte abraço.