Units & Measurements GPS Speed

GPS speed is your boat's speed over ground as calculated by the GPS receiver in your fish finder. It works by measuring how quickly your position changes between successive GPS readings — if you moved 100 feet in 10 seconds, the unit calculates your speed accordingly. GPS speed is available on any fish finder with a built-in GPS receiver, which is nearly every modern unit.

The advantage of GPS speed is availability and reliability. It requires no additional hardware beyond the built-in GPS and works in all conditions without maintenance. There are no moving parts to foul, no paddlewheel to clean, and no speed sensor to calibrate. For navigation purposes — estimating arrival times, planning fuel consumption, and monitoring cruising speed — GPS speed is perfectly accurate.

The limitation of GPS speed becomes apparent in current. GPS measures movement relative to the earth's surface, not relative to the water. When current assists or opposes your movement, GPS speed does not reflect the actual water flow speed across your hull and lures. A kayak angler drifting downstream at 3 mph in current with paddles up is not trolling at 3 mph — their lure is sitting motionless in the current relative to the water.

For most freshwater fishing in lakes and reservoirs without significant current, GPS speed is functionally identical to speed through water and works fine as a trolling speed reference. In rivers, tidal areas, and ocean fishing where current is a constant factor, a paddlewheel speed sensor provides the more useful measurement for bait and lure presentation.