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Britax Marathon 70-G3 Convertible Car Seat Onyx – Review and Sale

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Last modified:November 28, 2019


Britax Marathon 70-G3 Convertible car seat Onyx is one of the top selling convertible car seats on the market today. Find out why and where to get the best deal

Britax Marathon 70-G3 Convertible Car Seat Onyx

Britax Marathon 70-g3 Convertible Car Seat Onyx

The Britax Marathon 70-G3  Convertible car seat Onyx  has now been replaced with the more advanced Britax Marathon Clicktight Convertible. It is a top class convertible car seat which has taken many safety and comfort features into consideration. A convertible car seat that is suitable for infants from 5 -40 lbs rear facing and 20 to 70 lbs forward facing.

It’s height limit is 49 inches, however as children have different growth rates there is a chance that your child will outgrow the chair before reaching 70 lbs weight limit.  Below we will look more closely at why this is one of the top selling convertible car seats on the market and where you can get the best deal from.


Britax Marathon 70-G3 Features

Britax is a USA company whose brand name has become synonomous with the safety. What is more important than the saftey of  out loved one? The Britax Marathon 70-G3 convertible car seat onyx is designed with safety as it’s top priority. It uses special technology, called the Safe Cell technology to make sure that your child is protected in the event of  a crash. This technology ensures that when there is an accident a portion of the seat will compress allowing for the center of gravity to be lowered which counteracts the forward rotation of the child seat which normally pushes the baby into the front seat. This technology also helps to increase the protection on the baby’s head and body in the event of a crash.

The seat has steel bars integrated and an anchor that is energy absorbing in order to help reduce the forces in the event of a crash, it also comes with HUGS ( harness, ultra guard system) and side impact protection which diverts crash forces and EPP foam. Britax marathon 70-g3 convertible car seat

The Britax Marathon 70-G3 convertible car seat has multiple recline positions from both the rear and forward positions. There is also high-density foam and a plush cover equipped with a belly pad that matches. The five-point harness is tangle free and comes with the option to buy an insert for positioning  to ensure comfortable fit for smaller sized babies (sold separately)

This car seat is easy to install. The Latch connectors allow for fast, easy and secure installation and the push button makes uninstalling the car seat really simple.

There is no need to remove the seat in order to wash it. The cover of the car seat is easily removable. When choosing a fabric for your seat it’s advisable to choose one that is of velour or cotton fabric as the silver fabric of the Onyx heats up very quickly and can burn fingers when touching it.

It has a 7 year life span and a one year guarentee.

Product Features and Specifications

  • Rear Facing (lb): 5 to 40
  • Forward facing (lb): one year and 20, maximum 70
  • Child Standing Height (in): maximum 49
  • Child seated Shoulder Height: Rear Facing (in): 9 to 16.75
  • Forward facing (in): 12 to 16.75
  • Product weight (lb): 19.5
  • Product dimensions (in): 18 x 26 by 21
  • Seat Area Depth (in): 11 /11
  • Shoulder Width (in): 13
  • Harness Slot Heights (in) 8.5 / 9.5 / 10.5 / 11.5 / 12.5 / 13.5 / 14.5 / 15.5 / 16.5 / 17.5
  • Buckle Strap Depths (in) 5 / 7.5
  • seat made from 100% polyester.

The Marathon 70 has a deep seat pan allowing for more leg room/thigh support. It is a fairly large seat, hence it may not fit perfectly in a small car rear facing. It can be used on an aeroplane.

When buying the Britax Marathon 70-g3 car seat you will receive: Britax Marathon 70-G3 Convertible Car Seat, removable cover, comfort pads, infant body pillow, and user guide.

When researching the Britax Marathon car seat you will find numerous reviews out there, here are what some people have to say:

One reviewer said that this is the Rolls Royce of car seats and that they seriously regret not getting this seat for their first child. They mentioned the substantial padding that is incredibly thick and also raved over the convenient features that it carries.

Another reviewer said that after their husband was hesitant to spend too much money on a car seat, they researched this and purchased it. They said that if you’re looking for a seat that meets your safety standards, this is it.

Another reviewer said that you cannot put a price on your loved one and you can tell at first glance that this is a quality made car seat that is solid and safe.

Yet another reviewer reitteratted what was said above that it is big and heavy, so it may not be the best for air travel or small vehicles, rather go for the Britax Roundabout 55 for a small car.

P-Town Mama says in her review: ” The product quality is remarkably better than other brands. The ease of installation is priceless!! Hands down, the easiest car seat installation in 7 years of parenting.” Read the full review here

Troy Carrol reviews:” Its soft, comfortable, and safe. It was easy to install, just like all other Britax items.” Read more here


The Britax Marathon 70-G3  Convertible car seat Onyx  which is now the Britax Allegiance Convertible Car Seat is a high quality, functional car seat for parents of infants of all sizes. It’s a seat where saftey is it’s number one priority, its comfortable for baby to sit in and has many raving reviews. Its one of the top convertible car seats on the market and for good reason. It is on sale at the moment. Don’t miss out on the great price!


Rear Facing Installation using LATCH on the Britax Marathon 70-G3  Convertible car seat Onyx


Adjusting the Harness Height on the Marathon 70 -g3 Onyx

Rear Facing Installation using  lap/shoulder belt

Britax Marathon 70-G3 Convertible car seat Onyx is one of the top selling convertible car seats on the market today. Find out why and where to get the best deal