500 Miles of Testing
Three runners each completed a 16-week marathon training plan (50-70 miles/week at peak) while wearing two trackers simultaneously. We compared GPS tracks against a surveyed 400m track, heart rate against a Polar H10 chest strap, and training load metrics against lab VO2max measurements.
Top Pick: Garmin Forerunner 265
GPS accuracy was the best in our test — average error of 0.3% on measured courses (1.2 meters per kilometer). The multi-band GNSS receiver maintained signal through tree cover and between tall buildings where single-band watches drifted 5-10%. Heart rate accuracy averaged within 2 BPM of the chest strap during steady-state runs and within 5 BPM during intervals — the best wrist-based performance we measured.
Best Training Insights: Coros Pace 3
Coros's EvoLab training analytics rival Garmin's at half the price ($299 vs $450). The threshold detection automatically updates your training zones based on recent performance — we found its lactate threshold estimate within 3 BPM of our lab test. The battery lasted 38 days in daily use mode and 24 hours in continuous GPS mode, enough for any marathon.
Best Budget: Amazfit Cheetah
At $200, the Cheetah offers dual-band GPS, wrist-based heart rate, and Zepp Coach AI training plans. GPS accuracy averaged 0.8% error — good but not class-leading. Heart rate was reliable for steady-state runs (within 4 BPM) but struggled during intervals (up to 12 BPM off). The AI coach generated reasonable training plans that adapted to our recovery data.
GPS Accuracy Rankings (measured 5K loop)
- Garmin Forerunner 265: 5,003m (0.06% error)
- Coros Pace 3: 5,008m (0.16% error)
- Apple Watch Ultra 2: 5,012m (0.24% error)
- Amazfit Cheetah: 5,041m (0.82% error)
- Samsung Galaxy Watch 7: 5,065m (1.3% error)
- Fitbit Charge 6: 5,089m (1.78% error)
Heart Rate Accuracy During Intervals
Wrist-based optical heart rate sensors struggle during high-intensity intervals due to motion artifacts. For interval training, we recommend pairing any watch with a chest strap. The Garmin and Coros both support ANT+ and Bluetooth chest strap connections. For steady-state runs, wrist-based accuracy from the top three watches is sufficient for training zone monitoring.
Training Load Metrics
Garmin's Training Status and Coros's EvoLab both provide actionable metrics: acute load, training effect (aerobic vs. anaerobic), recovery time, and race time predictions. We found Garmin's race predictor within 90 seconds of our actual marathon time; Coros was within 2 minutes. Both are useful for pacing strategy.
Final Recommendation
The Garmin Forerunner 265 is the complete package for serious marathon training. The Coros Pace 3 delivers 90% of the features at 65% of the price. Budget runners should consider the Amazfit Cheetah but pair it with a chest strap for accurate interval heart rate data.
