Q: My 2001 BMW 740 IL Transmission Failsafe light came on when I put the car into drive

Your car has an electronically controlled transmission. If the controller detects a fault in the transmission it goes into failsafe mode to "work around" the problem. It might for instance skip a gear that it finds to be a problem when going through the upshift sequence. The only thing to be done is to have the codes read by someone with a BMW designated scan tool. The Generic tools that they use at the auto parts store won’t do the trick in this case. Once the technician knows what codes are stored in the system they can make a recommendation as to a fix. BTW: this probably has nothing to do with the tire change unless you had tires of the wrong size fitted. As for the clear fluid: some of the cars were equipped with a hydraulic suspension and the pump and reservoir for the system were in the trunk. Perhaps you are seeing a leak from that equipment. You can have these things checked out by a professional at your convenience by contacting Your Mechanic. They can send a technician to your home or office to check out your transmission warning and let you know what is the fix.