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.
Spec
Diono Radian 3RXT
Cybex Eternis S
Stage
Convertible
Convertible
Narrow (3-across)
✓
–
Extended rear-facing
✓
✓
FAA-approved
✓
✓
Lightweight / travel
–
–
How they differ
the Diono Radian 3RXT is listed narrow (roughly ≤17″) for fitting three across a back seat; the Cybex Eternis S isn't listed narrow
Published limits
Mode
Diono Radian 3RXT
Cybex Eternis S
Rear-facing
5–50 lb · 18–44 in
4–50 lb · 17–48 in
Forward-facing harness
22–65 lb · 30–57 in
22–65 lb · 28–49 in
Booster (with back)
40–120 lb · 40–57 in
40–120 lb · 44–57 in · 3+ yrs
Useful life
10 yrs from purchase
—
Diono Radian 3RXT: 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 + 22 lb minimum (printed manual requirement). No separate age requirement printed for booster mode. Expired over 10 years from date of PURCHASE (manual's own wording — most other brands print "manufacture").
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.
🚗 Need it before the due date or a trip? Amazon Prime's free 30-day trial includes free fast delivery and free returns — useful when a seat may not fit your car. Start the free 30-day trial →
Affiliate link — we may earn a commission on qualifying sign-ups, at no extra cost to you. Current terms are on Amazon's page.
As an Amazon Associate we earn from qualifying purchases, at no extra cost to you. Affiliate links never affect our specs or which seats we compare.
FAQ
What's the difference between the Diono Radian 3RXT and the Cybex Eternis S? The Diono Radian 3RXT is narrow for 3-across, extended rear-facing, FAA-approved for aircraft; the Cybex Eternis S is extended rear-facing, FAA-approved for aircraft. Both are convertible car seats.
Which fits three across, the Diono Radian 3RXT or the Cybex Eternis S? The Diono Radian 3RXT is listed narrow (~≤17″) for 3-across; the Cybex Eternis S isn't. Three-across fit still depends on your exact vehicle — confirm before buying.
Which is right for my car and child, the Diono Radian 3RXT 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: