You're three hours into a mountain ultra when your watch dies. Or worse—you're navigating a technical descent and your GPS drops signal under tree cover. Here's what separates the weekend warriors from the athletes who finish: a watch that actually works when it matters.

The Amazfit T-Rex 3 Pro is the most capable GPS sports watch under $400, built to survive the conditions that kill other watches.

Industry GPS That Locks Fast, Stays Locked Dual-band positioning across six satellite systems means you get accurate tracking under dense forest canopy, between canyon walls, and in urban corridors where other watches lose signal. The circularly-polarized GPS antenna connects faster and holds stronger.

Battery Life That Matches Your Training Up to 25 days of typical use, or 38 hours in full GPS mode. That's enough for a 100-miler without rationing features.

Built to Military Spec, Not Marketing Hype Grade 5 titanium bezel and buttons. Sapphire crystal display rated to 3,000 nits—readable in direct sun. MIL-STD-810H certified for shock, temperature extremes, and humidity. This watch handles what your training throws at it, from desert heat to alpine cold with dedicated low-temperature mode down to -30°C.

The BioTracker 6.0 sensor delivers accurate heart rate data even when you're moving hard over technical terrain. Pair it with the optional Helio Strap arm monitor for chest-strap accuracy without the chest strap. BioCharge monitoring adapts throughout your day, tracking how workouts and stress impact your readiness. Zepp Coach provides AI-generated training plans for everything from 5K to marathon, with real-time performance metrics including VO₂ Max, Training Load, and Training Effect.

Training: Pre-loaded offline maps with turn-by-turn navigation. Route planning with auto-rerouting. 180+ sport modes with smart recognition for 25 strength movements. Connect to power meters, heart rate monitors, and cycling sensors via Bluetooth.

Daily Use: Take calls from your wrist. Control music. Voice control through Zepp Flow. Storm alerts and high-altitude warnings for mountain training.

The Details: 10 ATM water resistance rated for 45m freediving and scuba. Quick-release straps (20mm for 44mm). Syncs with Strava, TrainingPeaks, Komoot, and major training platforms.

This isn't another overpriced GPS watch with features you'll never use. This is the tool that keeps working when conditions get real—backed by the battery life, GPS accuracy, and durability endurance athletes actually need.