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I believe the one driver told me 20%. There are also different rules for Grab Taxi versus Grab Car. Again if memory serves and the information shared by the driver was correct Grab takes 30 Baht for Grab Taxi bookings. I was picked up by a taxi the other day when I used the Grab App. He said he responds to GRAB taxi and Grab car bookings (rates are different) and also picks up customers as a meter cab as well.