Amazfit T-Rex 3 vs Garmin Fenix 8: Is the Garmin Fenix 8 Worth £620 More?
Amazfit T-Rex 3 or Garmin Fenix 8? We compare price, measured performance and real-world trade-offs to work out which is the better buy.
Daniel Reyes
The short answer
Amazfit T-Rex 3 at 68g against Garmin Fenix 8 at 10ATM, dive to 40m. Where they part company is who they suit: the Amazfit T-Rex 3 is the one to get if you are hikers who want Garmin endurance without Garmin pricing, the Garmin Fenix 8 if you are endurance athletes and anyone off-grid for days.
Price difference: The Amazfit T-Rex 3 undercuts the Garmin Fenix 8 by £620. That gap only matters if the extras below are ones you will actually use.
Specifications side by side
| Specification | Amazfit T-Rex 3 | Garmin Fenix 8 |
|---|---|---|
| Price | £279 | £899 |
| Our score | 4 / 5 | 5 / 5 |
| Display | 48mm AMOLED, 2000 nits | 47mm AMOLED |
| Battery | 21 days mixed use | 16 days mixed use |
| GNSS | Dual-band | Multi-band GPS, GLONASS, Galileo |
| Rating | 10ATM, MIL-STD-810 | 10ATM, dive to 40m |
| Weight | 68g | — |
| Sensors | — | HR, SpO2, barometer, depth |
How we tested
Every figure quoted above comes from the same protocol, run on every unit in this category. This one spent three weeks worn continuously, including overnight. Worn on the non-dominant wrist alongside a chest-strap heart rate monitor and a second reference watch, with all figures logged from the same sessions.
| What we measure | How |
|---|---|
| Heart rate accuracy | Beat-by-beat comparison against the chest strap across steady runs, intervals and weights. |
| GPS accuracy | The same 5km loop, twelve times, compared against a surveyed route. |
| Battery life | A fixed daily pattern — one hour of GPS exercise, always-on display, sleep tracking — run to empty. |
| Sleep tracking | Compared against a manual log of actual sleep and wake times over fourteen nights. |
| Durability | Worn through cooking, washing up and the gym, with no attempt to protect it. |
We deliberately do not report step counts, which every device on the market gets close enough that the differences are noise.
The honest caveat: Wrist-based heart rate is least accurate exactly where people care most — high-intensity intervals.
Where the Amazfit T-Rex 3 wins
- Three weeks of battery for a quarter of the Garmin price
- Genuinely rugged to military spec
- Dual-band GPS at this price
It also has the edge on the intangibles: twenty-one days measured, which no mainstream smartwatch approaches.
Where the Garmin Fenix 8 wins
- Sixteen days of measured battery life
- The best GPS accuracy we have recorded
- Genuinely rugged construction
And in day-to-day use, it traced a wooded valley route within four metres of a surveyed track.
The trade-offs
Neither is flawless. The Amazfit T-Rex 3 has one clear weak point — the app ecosystem is thin — and the Garmin Fenix 8 has its own: very expensive. Weigh those against how you actually work rather than against the spec sheet.
The value case
The Garmin Fenix 8 costs £620 more and scores 1 point higher — roughly £620 per point of difference. Whether that is good value depends on whether the extra buys you sixteen days of measured battery life, or only a number. In our testing it is the former, but only just.
If neither quite fits
Look at the Galaxy Watch 7 at £289 — it scored 4 out of 5 and suits Galaxy phone owners who want deep integration. If the budget will not stretch, the Fitbit Charge 6 at £139 is the cheapest option in this category we would still put our name to.
Living with each of them
On paper the gap looks like a specification difference; in use it is a habits difference. With the Amazfit T-Rex 3 the thing you adjust to is the app ecosystem is thin, and you do adjust — within a week it stops registering. Its display (48mm AMOLED, 2000 nits) is the part you stop thinking about. The Garmin Fenix 8 asks for a different accommodation: very expensive, against a display of 47mm AMOLED that holds up better than the number suggests. Neither compromise is severe. They are simply different, and which one you would rather not think about is a more useful question than which is objectively smaller.
Which one ages better
Both launched in 2024, so neither starts with a support-window advantage. The other half of ageing is the weak point you already know about: the app ecosystem is thin on one side, very expensive on the other. Weaknesses of that kind do not improve with firmware, so assume you are buying them for the life of the device and price the decision accordingly.
Our recommendation
For most people we would take the Garmin Fenix 8, which scored 5 out of 5 in our testing against 4 for the other. That said, a score is a summary, not a substitute for knowing what you need.
Buy the Amazfit T-Rex 3 for three weeks of battery for a quarter of the Garmin price. Buy the Garmin Fenix 8 for sixteen days of measured battery life.
Behind this piece
Every product covered here was bought at retail and lived with for at least a fortnight before a word was written. Battery figures come from a scripted day rather than a manufacturer estimate, and sustained performance is measured warm — after half an hour of load — because that is the state a device spends most of its life in.
Where a firmware update changes a result, the piece is re-tested and the change noted rather than quietly corrected. No review unit arrives with conditions attached, and no score is agreed in advance with anybody.
Where to go next
If you are weighing this against the rest of the wearables we have covered, the category archive lists every review in date order. The closest comparisons we have published are Amazfit T-Rex 3 vs Oura Ring 4: Which Suits You Better? and Amazfit T-Rex 3 vs Pixel Watch 3: Which Suits You Better?.
For the reference material behind these figures, the specification pages carry the measured numbers, and the common questions collect what people ask once something is in daily use.
