The lack of chemistry is what kills it.
I think the new Top Gear just confirms what everyone feared; when there's no chemistry between the hosts, it's cringe-inducing to watch. They might have the challenges and the brilliant cinematography but we may as well be watching an episode of Top Gear USA because of how little interest I have in the hosts (although I think TGUSA's actually may have had more chemistry).
Chris Evans is trying too hard to be funny and entertaining but he just comes across as an uncool nerdy bloke. Matt LeBlanc is fine but whatever lines the writers are giving him sound fake, unfunny and not something an American would say. I mean, the overuse of scripting was one of the criticisms of old Top Gear, so it isn't going to be improved with the change of hosts.
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"Show me a milkman in high heels and I'll show you a dairy queen."