I have acquired a set of NOS period BBS RS 16" wheels that were produced to fit XJS's and XJ40's, i.e. with a 5x120.65 PCD, 73.9 mm CB and 30 mm offset.
These are staggered RS020 and RS027, both 16" OD.
The RS020 (front wheels) are 7.5" wide and the RS027 (rear) 8.5".
The tyres that are mounted on the wheels are "new" Pirelli P5000 that are of course useless.
The dimensions are 225/45 R16 and and 245/45 R16 respectively.

Such tyre specs are obviously quite different from the dimensions that are recommended for the XJS (mine is fitted with 225/55 R16 tyres on 5-spoke alloys 7"x16" with an offset of 28.5 mm).

I wish to equip the rims with tyres that would correspond to the required perimeter i.e. 225/55 R16.
This is easy for the front wheels, 225/55 R16 tyres fit perfectly with the 7.5"x16" rim and there are plenty of good tyres proposed on the market.
The situation is more problematic for the rear 8.5"x16" wheels.
In principle the best fitment would be a 245/50 R16 tyre that would better exploit the increased width of the wheel, still keeping exactly the same perimeter and not fouling the wheel arch thanks to the 30 mm offset.
However, there are unfortunately no tyres on the market for that dimension, probably obsolete.

The only alternatives are to put a 235/50 R16 which has a smaller perimeter, or even further a 245/45 R16.
Or to put the same tyre than on the front, i.e. a 225/55 R16 maybe with an XL (extra load) spec, that would be somehow "stretched" (8.5" seems to be the limit of acceptability for a 225 wide tyre) but with the benefit of having the same tyres all around.

I am inclined to go to the 225/55 R16 route all around but I am wondering if that would put the rear tyres under too much stress, require an increased pressure (of how much?) and if that may worsen the ride of the car.

What is your opinion on these options?

 

Run a slightly taller and wider tire in the rear. I dont know about Belgum but here in the USA there are plenty of 255/50/ZR16 and 245/50 tires available.

The 225/55/R16 has a diameter of 25.8 inches

The 245/50/R16 has a diameter of 25.7 inches, imperceptible difference, but it would bug me because I would know.

The 255/50/R16 has a diameter of 26.1 inches. *** close enough to be imperceptible out back and may even look sporty as vehicles with larger rear tires often do.

Since the 245 is a wider tire, you will need an extra 3/8 inch of space on each side of the wheel well.

Since the 255 is a much wider tire, you will need an extra 1/2 inch of space on each side of the wheel well. Your speedometer will be off by 1%

Be sure to check this using the wheels you already have. Should be plenty of room in the XJS wheel well as long as the offset of your wheel is ideal. I've heard of people running 275 series tire on a stock wheel well XJS.

 

Just checked 255/50 R16.
That's a hell of a cost here! only Michelin Pilot Sport (450 Euros each) and Toyo Proxes R888 (300 euros each) available here...
There are a few 245/50 R16 available at a much more affordable price (around 80 Euros per tyre), unfortunately not the best brands, Kumho Ecsta KU 31 being the best option. That would mean different types/brands in front and rear as this Kumho does not exist in 225/55 R16. Woulsd that be OK?

 

It's not ideal in some situation but its often done. Street Racers often user a wider stickier rear tire that wears faster in the rear but offers increased traction. In your case I'd say it's OK since the tire is wider and would offer more traction. Make sure the two tires are the same type. For example make sure they are both all season performance tires or both high performance summer. It would not be ideal if the fronts were an all season GT tires and and the rear was high performance summer tire. I would mix brands, I wouldn't mix tire types.

 

Hi,
I know this is an old post, but if someone is looking for more information, I can tell you my experience.
I have a 1996 Jaguar XJS 4.0 Convertible.
With, BBS RS020 (ET30) Fronts, and BBS RS027 (ET30) Rears
The tyres are:
BFGOODRICH G-Force Sport Comp 2 (225/55 WR 16) Fronts
BFGOODRICH G-Force Sport Comp 2 (245/50 WR 16) Rears

When I fitted the wheels, I could turn the wheel lug nuts the minimum amount between 10.5 - 11 complete turns, but I was unhappy with the amount of stud inside the wheel lug nuts,
so I had new hub studs10mm longer than the original studs, they now are just protruding outside the open lug nuts, and I have peace of mind.

James