The smell will permeate. Mythbusters tried it with pig carcasses.

Dump the body in a self driving car. By the time they find it while digging out the car, you’ll be miles away.

Only if they're not a three thousand year old vampire. In that case YMMV.

I don't think it has ever been proved.

So, for example, the Krays were at their height atvthe same time there was a lot of road building in London, and people have suggested bodies went into the Chiswick Flyover:

There were rumours that victims of the Kray brothers, including Ginger Marks who shot Jack "The Hat" McVitie, were interred among the 100-tonne concrete support columns.

And the Bow Flyover:

The Bow Flyover in East London where legend has it that the body of Frank 'the Mad Axeman' Mitchell, one of the twins' victims, is buried in the concrete.

In fact, Jack "the Hat" McVitie may have suffered a similar fate:

Reggie stabbed Jack the Hat McVitie and the body was buried in the concrete foundations of a supermarket.

The problem I'd you'd need to demolish any structures to discover the truth, although given the age of them, some are being taken down these days so... Similar rumours emerged during the demolition of a bridge near Glasgow:

THE Kingston Bridge may give up a grisly 30-year-old secret in the next few weeks - the bodies of murdered criminals.

The bridge is widely believed to be a concrete tomb for several notorious gangsters who disappeared mysteriously in the late 60s.

Glasgow godfather Arthur Thompson, who died six years ago, is said to have ordered them killed and disposed of in the concrete piers of the bridge while it was under construction.

Now the piers are to be broken up and the truth might finally come to light, including the solution to the mystery of Archie McGeachy, who was 22 when he vanished from his Govanhill home in 1969.

Jack "the Hat" McVitie is still unaccounted for but there are other suggestions about his final resting place:

The body was never recovered, although in an interview in 2000 (which featured Reggie Kray giving a frank account of the activity of the Firm twelve days before his death) Freddie Foreman admitted to throwing McVitie's body from a boat into the sea at Newhaven, Sussex, although he was also reported to have been buried in a newly dug grave at Gravesend Cemetery in Kent.

Which takes us neatly back to the other suggestion - in a newly dug grave.

However, one of the people at the murder has confirmed McVitie was dumped at sea, even if the longer journey carried the risk of discovery, as described by Chris Lambrianou.

Also the police found a body under the patio at the home of an associate of the Krays. So the means of body disposal was likely pretty crude and simple.

It didn't work out well in this case