Probably a topic that has been thrashed before but I have a couple of options for tyres for my 2 1/2 SS and 3 1/2 Mark IV saloons.
Option 1. Buy a tyre as close as possible to the original specs as per the handbook. This would appear to be a 550-18 of 28.75" overall diameter giving the correct engine rpm vs 55 MPH roadspeed (as described in the handbook.
Option 2. Buy a larger tyre say 600/650-18 of overall diameter of 30.5". This might look better on the car and drop the RPM @ 55mph by 150. (Will the spare still fit I’m not sure.) Is this worth considering?
If you are planning some long trips and want to reduce engine revs to reduce interior noise then I would suggest retaining original tyre specifications. If your cars are anything like mine then wind noise is very much greater than any mechanical noise. Far better to attend to sun roof, door gaps, windscreen seals and trafficator slots than have problems with the size of your spare tyre.
Admittedly I am talking as an SS owner and the MkIV spare compartment is probably slightly better designed than mine but I have to stow my standard spec. spare deflated.
Yes . I’m sure the larger spare tyre won’t fit. You’ll also affect acceleration. You need to check different brands Some have amalgamated tyre sizes under "550"which may have been made using 525 moulds.
From experience engine noise is the least of the audible issues.
I bought a MKIV back in 1959 and when I went to collect it the R/F was flat. No probs. said the vendor and proceeded to extract the spare. It was 650-19 and it was REAL struggle to get that out because it was so thick. As it was my first car and a Jaguar I never really noticed the odd way it sat until next day. It had a 450-18 on the R/R, a 550-18 on L/R and 550-18 on the L/F plus the 650-19. Needless to say when I got it home my mother wanted to be taken for a drive. During this drive there were various twangs and the steering getting all peculiar. As I applied the brakes at our door the 650-19 gave up the ghost and collapsed. The ‘twangs’ were the spokes breaking.
So in summary if you do get it in the spare where carrier, you will have a helluva job getting it back out.