I guess you really need that tyre width when having to pass 1000+bhp to the road with rear wheels only!
19" (!!) x 15" wheels!
And while I am here, a bit more info on the car: https://www.stuttcars.com/porsche-models/917/30-can-am/
(clic on the link at the top to see all versions of the 917, it is also very interesting, especially that 917 Spyder with a 16 cylinders engine!)