I have a 2013 XF currently with 19 inch rims and 255/40zr19 tyres, they look great but transmit every bump and pothole with
bone jarring force, can I put smaller rims and thicker tyres on to give more cushioning especially for winter as it will be a bumpy
ride otherwise.

 

You can, but be sure they'll clear the brake calipers.

Hopefully this will be moved to the right forum - and there may even be answers about such smaller rims already there if you look.

 

Hi Thanks for all the help and assistance,
Here is what I found out.

My fitted tires OEM are 245/40R19 = 26.7"diam * 9.6" width, with 4" tire depth usefull site https://tiresize.com/calculator/

Tyre diameter 26.7" this is important to keep speedo reasonably accurate. So any tyre size/width and ratio must give very similar diameter.
Minimum inside width of rim to clear MY calipers is 15.25" on front and rear which I got using 18" rims with 225/50 R18 tires 26.9"diam * 8.9" width
Getting a much smoother ride and pot holes etc not as Jarring due to xtra tire depth?, this could be in part they are winter tires so not as hard as we head into winter (soon to be Arctic)
Speedo appears to be out at top end by about 3kms UNDER which is unusual as it should higher as tire is slightly higher diameter by 0.7% so travels further every revolution.

 

When I look at the tyres we used to fit to the old XJ saloons, there was no Summer/Winter split, they were used all year round, and had "sipes" in the tread pattern just like you now get with winter tyres. I think you're probably best just keeping the winter tyres on all year and putting up with the additional wear. They certainly won't last as long as the summer pattern tyres, but what the hell !