1. Top recommended engine rpm. If a tachometer is used, it will have a mark (red line) indicating maximum rpm. Some tachometers mark the red line with a colored sector. Others have two lines: the lower one marking the maximum allowable sustained engine rpm, the higher line indicating the absolute maximum rpm.
2. (RL) Tires with a red concentric line which were marketed in the late '60s and early 70s.