Baby Car Seats: Britax Marathon 70-G3 Convertible Car Seat, Onyx

Baby Car Seats: Britax Marathon 70-G3 Convertible Car Seat, Onyx
From Britax USA

The Britax Marathon 70-G3 convertible car seat accommodates children rear facing from 5 to 40 pounds and forward facing from 20 up to 70 pounds. The Marathon 70-G3 is purposefully designed and engineered to minimize the forward movement of your child's head during a frontal impact to reduce the risk of head injury with the following revolutionary Britax head safety technologies: base with safecell technology designed to compress in a crash, significantly lowering the center of gravity and counteracting the forward rotation of the child seat. Integrated steel bars strengthen the connection to the vehicle and reduce forward flexing of the child seat during a crash. Energy absorbing versa-tether features a two-point attachment and staged-release tether webbing to slow the forward movement and reduce the crash forces reaching the child. The Marathon 70-G3 also incorporates Britax side impact protection to contain the child and protect from the most severe types of crashes. The five-point harness is tangle-free with an fez-buckle system to retain the harness buckle in a forward position and harness holders that keep harness straps pulled to the sides to make boarding quick and hassle-free. Other features include a no-rethread harness and an easy-remove cover that also allows access to belt paths from the front of the seat, while the multiple buckle and recline positions, comfort pads, and infant body pillow provide premium comfort and positioning.Car Seats: Britax Marathon 70-G3 Convertible Car Seat, Onyx From Britax USA - vmarketingsite

Product Details

  • Amazon Sales Rank: #145 in Baby Product
  • Color: Onyx
  • Brand: Britax USA
  • Model: E9LJ51A
  • Released on: 2012-07-01
  • Fabric type: 100% Polyester
  • Number of items: 1
  • Dimensions: 26.00" h x 18.50" w x 21.00" l, 19.50 pounds


  • Base with safecell technology designed to compress in a crash, significantly lowering the center of gravity and counteracting the forward rotation of the child seat which normally propels the child toward the front seat
  • Integrated steel bars strengthen the connection to the vehicle and reduce forward flexing of the child seat during a crash
  • Energy absorbing versa-tether features a staged-release tether webbing to slow the forward movement
  • HUGS- Harness Ultra Guard System chest pads provides resistance to forward movement in the event of a crash
  • Side impact protection comprised of deep side walls lined with energy-absorbing EPP foam distributes crash forces, shields from vehicle intrusion, and contains the head, neck and body

Editorial Reviews

Packed with comfort and safety features, the BRITAX MARATHON 70-G3 Convertible Car Seat is designed with 360-degree protection to keep your child secure in the event of a crash. It offers high-density comfort foam for extra support, a cushiony cover, and an EZ-Buckle system for convenience when harnessing your child. The MARATHON 70-G3 accommodates children rear facing from 5 to 40 pounds and forward facing from 20 to 70 pounds.

Convertible Car Seat
At a Glance:
  • Side Impact Protection system with energy-absorbing foam diverts crash forces
  • Base with SafeCell Technology compresses to counteract forward momentum
  • Energy-Absorbing Versa-Tether anchors seat and slows forward movement
  • Integrated steel bars reinforce seat stability
  • Tangle-free, no-rethread, five-point harness provides a secure fit
Age/Weight Requirements:

5 to 40 pounds rear facing; 20 to 70 pounds forward facing


Limited one-year warranty

MARATHON 70-G3, Onyx Product Shot

With the Side Impact Protection system, energy-absorbing foam helps divert crash forces. View larger.

MARATHON 70-G3, Onyx Product Shot

SafeCell Technology helps to minimize the risk of head injury during a collision. View larger.

MARATHON 70-G3, Onyx Product Shot

The EZ-Buckle System maintains the harness buckle in a forward position to prevent your child from sitting on it when boarding. View larger.

Side Impact Protection System Diverts Crash Forces

The BRITAX Side Impact Protection system includes a rigid shell with deep sidewalls lined with energy-absorbing EPP foam to contain your child's head and body and divert crash force energy while creating a barrier against possible intruding objects. This system also protects your child's head and chest.

SafeCell Technology for Advanced Protection

The base of the MARATHON 70-G3 includes SafeCell Technology comprised of SafeCells, an engineered material designed to compress in a crash. This compression lowers the center of gravity to counteract forward rotation of the seat, helping to minimize the risk of head injury during a collision.

Located in the sides of the seat shell, integrated steel bars strengthen the connection to the vehicle and prevent forward flexing of the seat during a crash to reduce the risk of head injury.

In the event of a sudden stop, the Energy-Absorbing Versa-Tether strap restricts forward movement and reduces crash forces with patented staged-release webbing and a 2-point attachment. This webbing securely anchors the top of your child seat to your vehicle in either rear- or forward-facing positions.

No-Rethread Harness System and Comfort Features

The tangle-free, five-point harness allows for multiple height positions and can be adjusted without dismantling the harness or removing the car seat from the vehicle. Two buckle positions provide a snug, comfortable fit as your child grows. The seat's harness holders prevent the straps from getting in the way when you're putting your child in the seat, while the EZ-Buckle system maintains the harness buckle in a forward position to prevent your child from sitting on it when first entering the seat. Rear- and forward-facing recline options, as well as high-density foam cushioning, help your child relax on the journey. And the plush cover comes with matching comfort pads and an infant body pillow to keep children of all ages comfortable and secure.

Easy Installation and Airplane-Approved Design

The MARATHON 70-G3 takes the guesswork out of installing your car seat thanks to a range of convenience features. An anti-slip, contoured base grips and protects the vehicle seat. Premium lower LATCH connectors make for a quick, simple, and tight installation, while the push-button allows you to easily uninstall the car seat. And easy-to-access built-in lock-offs provide a secure installation when using the vehicle lap and shoulder belt.

The MARATHON 70-G3 has a durable cover that can be removed for easy cleaning without uninstalling the car seat. It's also approved for airplane travel.

Product and Warranty Information

The BRITAX MARATHON 70-G3 is made in the USA, has a seven-year life, and is backed by a manufacturer's limited one-year warranty.

What's in the Box

BRITAX MARATHON 70-G3 Convertible Car Seat, removable cover, comfort pads, infant body pillow, and user guide.


Click here to compare BRITAX car seats.

OnyxWaverlyCowmooflageAzaleaChili Pepper
OnyxSilver BirchAztecCaitlinZebra
OnyxAnnaOpus GraySereneZebra
Trademark Information:


SafeCell Technology®

Energy-Absorbing Versa-Tether®

Customer Reviews

Most helpful customer reviews

459 of 477 people found the following review helpful.
4Britax vs. Recaro (Britax Good but I prefer Recaro) 

I have one each of the Britax Marathon and the Recaro ProRide for our two vehicles. I studied and agonized over the decision on which car seat(s) to get when I purchased them several months ago. There are a plethora of reviews that pit the Britax against the Recaro, so to save you reading time, I'll keep my review brief:

Pros of Both:
-Sturdy, High Quality
-Great padding
-Plenty of adjustment for a good secure fit (and our little girl is definitely behind the growth curve in weight: 1 year old and 17 pounds!)

Cons of Both:
-This is probably vehicle dependent, but if installing as rear facing then sometimes difficult to get it level because of how many vehicle seats slope down. Ultimately, in our Camry I followed manufacturer recommendations and used a tightly rolled towel underneath to achieve the right placement.

Reasons I like Recaro (I'll qualify this by saying both are good, this is just a preference and am probably splitting hairs):
-installation slightly easier due to nature of the latch belts.
-harness release is much easier to find and just seems to make more sense (ergonomically) based on how your hands work when you are reaching in the vehicle.
-seat seems less bulky and easier to transfer child in and out of.
-harness is easier to adjust and fit seems more snug (note on the Britax, fit is FINE, but we can't get any tighter so ideally it could be a hair better.
-shoulder pads stay in place better
-easier for child to see out of (keep those eyes busy!)

So, these reasons may not seem like a big deal...which is what I would have said when I was researching. I will close with saying, both are good, and I did not bother to return the Britax, BUT, if I had to go back several months and do it all over again, I would just get two of the Recaro ProRide models.

Happy researching!

196 of 220 people found the following review helpful.
5Another great seat from Britax 
Since 2001 I've been hooked on Britax car seats. The quality and ease of use of their seats puts them above their competition. When I was presented with the opportunity to review the Britax Marathon 70-G3 I jumped at the chance to see how it differed from the Marathon 70.

We own a Britax Marathon 70 so I decided to put it next to the Marathon 70-G3 for a side by side comparison. For the most part the seats are practically identical with a few small differences. The harness adjustor strap's release mechanism is the same, and I still find it awkward when compared to the Marathon and Roundabout 55 classic seats. Instead of pressing down, you need to press up. After nearly 11 years of dealing with seats that have easier releases, I still find this to be awkward. Harness height adjustment on this seat is a snap compared to the seats where you need to rethread the harness. You simply have to squeeze and pull (or push) to adjust the height of the harness straps. The ease of use for this function is on par with the simple adjustment of the back on a Graco TurboBooster. Parents who travel frequently and find themselves lugging their child seat through airports will appreciate the added touch of an area to clip the versa tether strap to be used as a carry handle.

The difference between the Marathon 70 and 70-G3 lies in the versa tether and the harness buckle. The versa tether on the new Marathon 70-G3 is energy absorbing and works to reduce forward movement and rotation in a crash. The seat also has the EZ-Buckle System which keeps the harness buckle in a forward position and keeps your child from sitting on the buckle. I can tell you that this feature is quite nice. I have been using a Roundabout 55 Classic with our nine month old and I can't tell you how often she ends up sitting on the buckle and freaking out while I try to pull it out from under her. The new design really works nicely to keep the buckle out of the way when loading the baby into the seat. The design reminds me of the harness buckle on our B-Agile stroller.

Installation of this seat was a snap. I installed it both rear and forward facing in my Toyota Sienna and it was easily and securely installed in both positions. Because the seat is so large, it is possible that this seat may not fit well in smaller cars when used in the rear facing position.

Overall, this is a great quality seat that will keep your little one safe, comfortable and secure for many years. In the nearly eleven years that I've been using Britax seats, I have found each of them to be worth the price.

77 of 92 people found the following review helpful.
5Fantastic Car Seat that fits great in my car! 

This car seat makes the other two I have seem like crap. I was given a Safety First Alpha and Omega, it's ok but it's hard to install correct and currently it is slanting to the left, I have to figure that out. I also bought a cheaper $40 Cosco Scenera from Walmart as a back-up car seat to go to my mom's. It is hard to get tight and it moves a lot front to back.

I had looked for a car seat for a couple weeks. I read review after review. Anytime one had a negative review I debated whether I should get that car seat or not. I started getting a bit paranoid about getting the perfect car seat for my daughter. I mean she is my most precious cargo.

First off, SIZE: my HUGE concern when getting a convertible seat was that it would fit in my Mazda 3 rear-facing. Many reviews of car seats I was interested in mentioned how huge they were and how difficult to fit in a smaller car. Some reviews mentioned how well this (Britax) fit in cars so I decided to give this one a try (because it also had great reviews based on safety and comfort etc...)It fits beautifully. If you install it directly behind a seat you'll probably have to move the seat up just a bit, but not bad. I like to put the seat in the middle though because that is the safest place. It fits perfectly. I can move the front seats back as much as I like. This was a huge selling point for me, and I know that when I was reading reviews it was something I was specifically looking for. I want to keep my daughter rear-racing as long as I can, so I needed it to fit nicely. I also love that this seat has a higher rear-facing weight limit of 40 lbs.

INSTALLATION: It took me a little bit at first but that was only because I was reading the manual carefully and familiarizing myself with the parts of the seat and the parts in my car. It was actually quite easy though. And now I could do it pretty fast for the second time. I used the LATCH system and it was pretty straightforward. But with every other car seat I have had I have used the seat belt. That way didn't look bad either. What I really liked about installing it with the seat beat route is that it has a locking mechanism under the car seat fabric that actually will hold the seat belt in place. You won't have to use those metal locking clips. The best part was after it was installed the car seat did not move! AT ALL! It didn't move side to side or even front to back it was solid. Another thing that I absolutely love about this seat is that I didn't have to use a stupid pool noodle to make it level. That always seemed lame to me that so many car seats are level on the floor, but most vehicles seats are slanted. Adding a pool noodle or rolled towel has always bugged me.

I love the material, it really feels nice and comfortable. The seat is also padded nicely. The velcro on the sides is nice because it holds the straps up so they don't get tucked under my daughters bottom. I also love how there is padded material that goes behind the buckle. This is nice because the straps won't dig into her chubby little legs.

I only have one negative thing to say, and that is more about the instruction manual. When it mentions the tersa tethering, I was a bit confused. I didn't understand based on the manual how to tether rear-facing. It was very vague. One thing, is you don't have to tether rear-facing it's optional (the manual made it sound like you had to) the other thing I thought is I needed an actual tether thing on the floor (like the one on the backs of seats). But you can tether using anything on the floor of your car that is bolted down. I used part of the track to move the front seat forward and back.

I love this seat and I would recommend it to anyone!

See all 847 customer reviews...

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