Jehu was a Biblical King, renowned for his aggressive chariot driving. When I was eight or
nine, autos were not numerous: you had to be well off to afford one. Thirty miles an hour
was considered fast. And when you were doing thirty, anybody who passed you was
"Driving like Jehu!" You might later exclaim that they were "Going forty mile an hour!"
And if you wanted to really exaggerate, because nobody would take it serously
"They were going like sixty!" An obvious lie, because few passenger cars were capable of it.