I have always just thought that the series 1 & 2 XJ cars fitted Dunlop as OE. I always thought up until '81/'82 when they introduced the HE version of the XJ12 they fitted Dunlop ER/70VR15 SP Sport.
However i have been looking at a selection of fitment guides from period and they all list the XJ6 as fitting ER/70VR15. but the 12 cylinder cars it lists as 205/70VR15.
Now a ER/70VR15, is actually a 205/70VR15. I think Dunlop just wanted to add some mystic by using the ER instead of 205. Pirelli were making low profile tyres as well in 1968, they were making a CN36 which Porsche used. then in '71 they made the CN12,
I am wondering if the V12 cars might not have fitted Pirelli instead of Dunlop?
Pirelli introduced the P5 in the later '70s. I know this was an OE tyre on the XJ saloons, and Pirelli have also already made the 225/65R15 P5 for the XJ40. I am just wondering if you chaps might be able to clear up some of this detail?