You have seen the stat. Some percentage of automotive leads arrive outside business hours, and it is always large enough to be alarming.
Published claims run from 35% to 65%, and the number tends to correlate with what the publisher is selling. We could not find a primary source for any of them.
What we will say
A lot of them come in at night. That is as far as the evidence goes, and it is far enough, because no dealer needs a study to know that Sunday evening exists.
A lot of them come in at night. That is the whole defensible claim.
Why the exact number was never the argument
Speed to lead is the wrong frame anyway. Stores that take leads seriously already answer them, and answer them fast. A vendor who opens by telling a dealer his response time is bad is telling him he has never been in a store.
The real constraint is not response time, it is the hours that cannot be staffed at a price that makes sense. A body on nights and weekends costs more than the cars he would sell, which is precisely why that shift stays uncovered in almost every store in the country.
That argument does not need a percentage. It needs a payroll calculator, and every dealer already has one.