Both are convertible car seats — here are their published manufacturer specs side by side.
This compares published manufacturer specs — not safety or fit advice. The seat's manual, your vehicle's manual, NHTSA's tools and a certified CPST are the fit-and-safety authority; always check them before installing.
As an Amazon Associate we earn from qualifying purchases, at no extra cost to you. Weight ranges are the manufacturer's published limits across all modes.
How they differ
the Cybex Eternis S lists extended rear-facing (a higher rear-facing weight limit, ~40-50 lb); the Britax Allegiance doesn't
Published limits
Mode
Britax Allegiance
Cybex Eternis S
Rear-facing
5–40 lb
4–50 lb · 17–48 in
Forward-facing harness
20–65 lb
22–65 lb · 28–49 in
Booster (with back)
—
40–120 lb · 44–57 in · 3+ yrs
Useful life
7 yrs from manufacture
—
Britax Allegiance: Harnessed seat only (no booster mode). Rear-facing 5–40 lb, forward-facing 20–65 lb; the manual's overall height limit is 49″ or less. Discontinue use once the seat is older than seven years (see the Manufactured Date Label).
Cybex Eternis S: All-in-one (rear-facing → forward-facing harness → high-back booster; no backless mode found in manual). Forward-facing also requires at least 1 year old. Useful life is set by the printed "Expires On" date (no printed number of years in the manual).
That is the record of the brand — not a verdict on these models. A recall names specific models and manufacture-date ranges, and a seat missing from the record is not "recall-free": check nhtsa.gov/recalls with the model and date of manufacture on your seat's label. Source: NHTSA, retrieved 2026-09-05.
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FAQ
What's the difference between the Britax Allegiance and the Cybex Eternis S? The Britax Allegiance is FAA-approved for aircraft; the Cybex Eternis S is extended rear-facing, FAA-approved for aircraft. Both are convertible car seats.
Which is right for my car and child, the Britax Allegiance or the Cybex Eternis S? That depends on your specific vehicle and child — this page compares published manufacturer specs only, not fit or safety. Check the seat manual, your vehicle manual, NHTSA's car-seat finder and ideally a certified CPST before deciding.
Citing these specs? Go ahead — published manufacturer data, last verified 2026-06-11. Copy a ready-made reference: