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Britax Marathon 70-G3

Est. $250
February 2014
by ConsumerSearch
Britax Marathon 70-G3

Best convertible car seat

  • Many safety features
  • No-rethread harness-height adjuster
  • High weight ratings
  • Easy to clean
  • May be bulky in some cars
  • Short shell

Bottom line

The Britax Marathon 70-G3 combines impressive safety with ease of use, parents and experts say. A steel-reinforced frame and tethers that work both rear- and forward-facing earn praise. Most reviewers say the seat is easy to install and adjust, and nearly all say it's a comfortable ride for little ones. But the seat might be too bulky for small cars, and its short shell means tall children may have to ride forward-facing sooner than parents would like.


Known for safety features. The Britax Marathon 70-G3 has a base with cells that compress in a crash, lowering the child's center of gravity and reducing his or her forward thrust. Integrated steel bars strengthen the connection to the vehicle and reduce forward flexing in the seat, also helping to keep the seat stable. Deep, rigid sides with energy-absorbing foam provide side-impact protection for the child's head and chest, and energy-absorbing tethers stabilize the car seat from the top in both the forward- and rear-facing positions. The Marathon's five-point harness is also equipped with chest pads that keep the top restraint of the harness in place over the child's chest for maximum protection in a crash. However, the seat lacks the more advanced true side-impact-protection head wings that pricier Britax models include.

Ease of use

Simplicity is the name of the game. Reviewers praise the Britax Marathon 70-G3's no-rethread harness-height adjuster, which allows parents to change the harness height from the front with one hand while the seat is installed. The seat has thick harness straps that resist tangling and a long, two-position crotch strap, making it easier to secure larger children. However, a few parents complain that the chest pads on the straps can be difficult to use, particularly on smaller babies. The seat installs easily with push-button LATCH connectors that don't require as much digging in between the seats. The Marathon 70-G3 is also equipped with lock-off fasteners that keep the seat belt tight around the seat when latch connectors are in use, though experts note they can be hard to access. The seat's cover is also easy to care for -- owners can remove it in one piece while the seat is installed. The cover is sturdy and can endure repeat washings, reviewers say. The Marathon 70-G3 receives 4 stars for overall ease of use in National Highway Traffic Safety Administration (NHTSA) ratings. Testers say the seat labels are incomplete and installation can be tough for those with larger hands, but praise the clear instructions.

Lifestyle features

Comfy, but may not be best for tall kids or small cars. Reviewers love the Britax Marathon 70-G3's plush feel. They say the seat is deep and roomy, and that the protective side wings provide a resting point for sleepy heads. The seat offers rear- and forward-facing recline positions to keep kids comfortable. It can be used rear-facing for children from 5 to 40 pounds and forward-facing for children up to 70 pounds, and has an overall height limit of 49 inches. However, experts caution that the seat's shell is relatively short, meaning parents may not be able to keep tall children rear-facing as long as they may like -- children must be turned forward-facing when there is only an inch of shell remaining above their heads. Some reviewers complain that the Marathon is harder to fit in smaller vehicles or when more than one car seat is used. Optional accessories include an attachable cupholder (Est. $15).

Customer service

Few service complaints. Britax receives solid customer-service reviews across the web. The Britax website offers comprehensive installation instructions for all models, but you must know the date of manufacture to access the information. All Britax car seats have a one-year limited warranty. The company must authorize all returns. The seat expires and should not be used after seven years. Britax recommends seats be replaced after a "moderate to severe crash" and lists specific criteria on its website.

Where To Buy
Britax Marathon G4.1 Convertible Car Seat, Onyx

Buy new: $289.99 $232.00   8 Used & new from $181.26

In Stock. Eligible for FREE Super Saver Shipping


Our Sources

1. Baby Bargains

"Baby Bargains" covers the Britax line in general and the Britax Marathon specifically. Authors Denise and Alan Fields consider performance, ease of use and the brand's track record to come up with their overall rating.

Review: Car Seats: Picking the Right Child Safety Seat, Denise and Alan Fields, Tenth Edition, 2013


This review covers safety, best price, comfort, ease of adjustments, ease of installation, dimensions, customer reviews, washing seat covers and pros/cons with detailed photos. This blog also compares the Marathon with other car seats, including the Graco My Ride 65.

Review: Britax Marathon 70 Convertible Car Seat Reviews, Editors of, Not Dated


The Britax Marathon 70-G3 receives a rating of 4.7 out of 5 stars based on more than 840 owner reviews. Users say they love the seat's plush comfort, thick harness and easy-to-install LATCH tethers, among other features. Most praise the fit in their car, but some say it's too cramped for older children.

Review: Britax Marathon 70-G3 Convertible Car Seat, Contributors to, As of February 2014

4. tests 27 convertible car seats. The ratings are reliable and unbiased, and they include editors' highs and lows for each seat -- helpful for parents who want a quick summary of each seat. However, there are a couple of discontinued models listed, and it's unclear when the tests were performed.

Review: Convertible Car Seat Ratings, Editors of, Not Dated

5. National Highway Traffic Safety Administration

The Britax Marathon 70-G3 earns 4 stars out of 5 for overall ease of use in the NHTSA's tests. The Marathon is penalized for poor labels, receives 5 out of 5 stars for evaluation of instructions and securing the child, and 2 to 3 out of 5 for installation features.

Review: Child Safety Seat Ease of Use Ratings, Editors of National Highway Traffic Safety Administration, As of February 2014


The Britax Marathon 70 makes Hollie Schultz's list of the five best convertible car seats. This review covers the seat's features at length, but doesn't mention any testing. She particularly raves about its high weight rating of 70 pounds, saying it may be the only car seat you ever need.

Review: Britax Marathon 70 Convertible Car Seat Blue Print, Hollie Schultz, Not Dated


The Britax Marathon 70 is at the top of Heather Corley's list of the eight best convertible car seats. She praises its ease of use and safety features. She seems to have tested the seat personally, and really appreciates the extra safety feature of the top tether.

Review: Top 8 Convertible Car Seats, Heather Corley, Not Dated


Michelle Horton praises all of Britax's 3G seats, which all have SafeCell technology in the harnesses and easy-to-use buckles. She also likes the energy-absorbing foam, ease of installation and no-rethread harness, but does not distinguish between seats.

Review: Babble Blogger Favorites: Infant and Toddler Car Seats, Michelle Horton, Nov. 12, 2012


Melanie Rosen notes that the Marathon 70 can be pricey, but says the seat's safety features and quality make it worth it. She praises the steel-reinforced frame, rear-facing tether and high height and weight limits, too. But the seat may be too big in small cars, she notes.

Review: 17 Convertible Car Seats with Extended Rear Facing, Melanie Monroe Rosen, Not Dated


The Britax Marathon 70 is one of the highest-rated convertible car seats here, with an average rating of 96 out of 100 based on more than 60 reviews. Owners say it's really great for long car rides, praising it for its outstanding comfort for children.

Review: Britax Marathon 70 Convertible Car Seat Reviews, Contributors to, As of February 2014


The Marathon 70 earns praise for its solid safety features, including its ability to lower a child's center of gravity in a crash. Yang compares it to wrapping your child in bubble wrap. She also likes the high weight limit and two buckle positions, which contribute to the seat's longevity.

Review: Top 10 Car Seats, Sarah Yang, Not Dated


The Marathon is's top 2012 convertible car-seat pick because of "durability, looks, and ease of use." Editors note the seat can be pricey, but the site's parent contributors say it's worth it for the additional safety it offers their children.

Review: Top 15 Baby Products of the Year: Moms' Picks, Editors of, Not Dated

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