Jumbo Visma car was about 3m at one stage (helicopter shot).
According to the commentators, there is a mild effect at 10m and quite a considerable effect at 5m.
[You didn't have to quote me 3 times! I'm flattered though
]
3metres - to me - is unacceptable. I don't recall seeing that in a televised race*, it would certainly ring alarm bells. I watched the highlights of this one, and probably not in great detail. Mostly saw riders taking corners at "brave" speeds, and gurning the last 400m to the finish.
I haven't seen the relevant regs, but that feels dodgy just on safety grounds. What possible justification does the DS have? They're all wearing radios - so no need to shout at your rider - and anyway the team car's prime function is mechanical backup.
Last time I read up on this 10metres was too far back to feel a difference, but speeds do go up, and aero studies/simulations get better.
*Worth watching Francesco Moser footage from his Giro win - note the motorbikes!