How does this seats maximum harnessed height compare to the Britax Frontier?

Hi Caroline, thank you for your inquiry, the major difference with these two seats is that the Frontier measures by weight where the Maxi Gaurd is measured by height, so really I would look at how your child grows if he is taller than expected but lighter then the Frontier maybe more suitable and so on, however, I would recommend visiting your local Baby On The Move Team and possibly trying both in your car as well as with your child to see which will last him the longest. We hope this helps.  For further information on this or any of our other products available, if you follow the link https://www.babyonthemove.co.nz/store-locations/ this will take you to our online store finder. Here you can find contact details for your nearest Baby On The Move Store where one of our team experts will be happy to assist you in your product search and help answer any further questions you may have. We hope this has helped with your query.