We charge tacho on all our aircraft, except the one that has an airborne time meter. Tacho is the fairest way and cannot be "massaged" like chocks time. The club members, whether students or PPLs, pay far less per minute whilst taxying and are therefore less likely to rush. On the other hand, if they fly fast cruise (high rpm) they pay more per minute but are charged for less minutes because they get to the destination quicker. The biggest drawback for us is that the rates look very expensive to the uninitiated.....................