In the Posthumus & Tremayne book Land Speed Record, the quote from the late Art Arfons about the December running in that year on Bonneville was "Slick hit 552 mph, before a big off-course moment, he decided enough was enough". The car was running solid (tireless) wheels on the salt which had been modified after some November running when speeds were "around 300 mph".
Any 'talk' of 650 mph peak would have been to keep sponsors happy, because the target average speed was 622.407 mph.