There are several useful shortcuts in this course, but the video shows the best one!
Turn around and go reverse inside the tunnel, then turn back around and continue forward. Right before you reach the end of the tunnel, drop down on the left side. If you do this correctly, the lap will change before you land, and you can drive back up the tunnel to do it again.
Alternate Shortcuts
1. When you reach the section that you can see the bridge to your right, jump over the guardrail.
2. When you are taking the bridge, veer off to the left as far as possible without falling into the water.
3. Inside the tunnel, cut time off by driving up the hairpin curve and cut the corner very short.
Mario Kart 64 Shortcut Video Walkthrough Series for Nintendo 64