What is the test for the presumption of wrongful death in a motor vehicle accident?

'Although there is no room for the presumption where the [172 Cal.App.2d 513] driver or other person whose claimed negligence is at issue himself testifies to his actions at the time involved (Speck v. Sarber (1942), 20 Cal.2d 585, 587-588, 128 P.2d 16), the rule is established that if such person be deceased or unable to testify by reason of loss of memory, the fact that other witnesses for the parties testify fully as to the acts and conduct of the allegedly negligent person does not deprive the party relying on the presumption of the benefit thereof unless the testimony which he himself produces 'under circumstances which afford no indication that the testimony is the product of mistake or inadvertence * * * is wholly irreconcilable with the presumption sought to be invoked.''