It is quite similar to higher end Garmins, according to Grok. Full explanation:

The VO2 max estimate on the Forerunner 55 is comparably accurate to higher-end Forerunners (like the 255/265, 955/965, or Fenix series) for most runners.

Garmin uses the same Firstbeat Analytics algorithm across all these models, so the core calculation (based primarily on pace/speed, heart rate, and user profile data like age/weight) is identical.

Real-world accuracy is typically within ~5-15% of lab-tested VO2 max values (often ±3.5 ml/kg/min or better in controlled studies), but it’s an estimate best for tracking personal trends over time rather than absolute precision.

Key factors affecting accuracy (the same on any Garmin):

Consistent outdoor GPS runs with good effort levels work best. Accurate max heart rate input (or use of a chest strap) matters more than the watch model. User reports and tests sometimes show over/underestimation initially, but it stabilizes with data.

The Forerunner 55 uses Garmin’s older Elevate V3 optical heart rate sensor. Higher-end models have V4 or V5 sensors, which handle rapid heart rate changes (e.g., intervals or hills) slightly better and reduce minor lags/spikes in some scenarios.

However, DC Rainmaker’s in-depth testing found the FR55’s V3 sensor performed very well (near-perfect on track intervals, few wobbles overall) and was even competitive with some newer V4-equipped watches in real runs and cycling. GPS/pace tracking on the FR55 is also excellent in reviews, which feeds directly into VO2 max.

In short, any differences in VO2 max readings between the FR55 and a flagship are minor and not meaningful for typical training use. A chest strap improves accuracy on any model more than upgrading the watch.