Mercury - Fuel Flow Gauge Dual Scale White Face 2-1/8" (52 mm) 79-8M0069416. Overview. The Mercury Fuel Flow Gauge (79-8M0069416) helps you keep an eye on how much fuel your engine's using in real time. It shows flow rate on a dual-scale face - both gallons and liters per hour - so it's easy to read no matter what units you prefer. The white face stands out well in sunlight, and the gauge fits a standard 2-1/8" (52 mm) dash hole. It's built tough for marine use and pairs perfectly with other Mer