So I have an 86 XJS and I want wider rear tires. Im having my factory rims widened and I need to know how far to go. They can only add to inside of rims so I will need spacers as well. Id like 300s but I think thats pushing it a bit. I dont want to add fender flairs if possible.

 

Not positive but I think the max in the rear stock is 245. I think 245/45r17
Not sure if 245 or 235 are easier to find or if one is a better option price-wise.

There are a couple of guys around here with aftermarket wheels and tires, hopefully they will chime in.
I've got 225/60 on wires, so this isn't really my department lol

 

i dont think any thing wider than 245/255 will fit with factory wheel arches!

i have 295/35/18 rear tires and and had to wide body the rear flares about 4" each side(lot of work).

 

Did you widen the whole rear panel, add flairs and cut the arches? Looks good.
I tried rolling the arches on my parts car with a bat. It worked good on my old Dodges but not so much on the XJS.

 

I am running 255/35/18 on the rear with zero issues ( XK8 Flute alloys )

225/40/18 on the front with a 25mm hubcentric spacer

BB

 

i've got 235/45-18s on the rear (XJR asteroids) there is a slight rub of the outboard side of the tire on the inside of the fender at full compression. but this may be due to the large-ish sidewall on these tires compared to what others run on 18 inch wheels.

also note, i believe there is a difference in the pre-facelift and facelift fenders, allowing for a wider tire in the facelift models.