Our Jaguar XF's came with size 245/40 tires on the 19 inch wheels. This is somewhat of a low profile tire and I feel a higher profile or "aspect ratio" as it is technically called would obviously create a more softer/comfortable ride. Has anyone purchased a higher aspect ratio tire on their XF? If so, I'd appreciate feedback on the size of the tire you bought and your thoughts.

 

Yes indeed, and it does make a noticeable improvement. I did some modifications in steps. First I installed the Comfort Suspension package from Spires which included 15% softer springs and custom valved dampers. I drove around for about a month like that and it made some improvement. Then I bought a full set of the 18" Vela wheels from Jaguar and installed 235/50r18 Goodyear Eagle tires. That's about 3/4 inch more sidewall than the original 245/40r19s. I believe I actually got more improvement from the change in tire size than from the suspension upgrade. But the combination is a major improvement. Its still nowhere near the magic carpet ride that was my old XJ8, but is now tolerable on the asphalt cattle trails that the City of Fort Worth calls streets and I think slightly better than most other conventionally sprung luxury sedans of similar size (most everything is a "sport" sedan nowadays). I also think it looks better.

 

Yes, but as pdupler says, you will need new wheels.
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Jaguar - it's not an automobile, it's a Motorcar
Current: '15 XF (X250) Portfolio AWD 3.0 (it's aubergine...)
Past B: '08 S-Type 4.2 "Satin Edition" (250.06 whp / 259.67 torque)
Past A: '05 X-Type 3.0/auto Jaguar Racing Green

 

So I need to switch to 18 inch wheels? No one makes 19 inch tires with a size greater then 40 (stock size is 245/40)? Like 245/45 or 245/50?

 

I replaced the factory 54 pound 18" rims and tires on my 99 XJR with 16" inch factory rims from an XJ8 and 215-65-16's weighing in at 42 pounds. Same tire diameter in both cases. Went from a 13.4 quarter mile time to 12.9 at 107mph. NASCAR runs 200mph on 15 inch wheels.

 

The problem is that the overall diameter grows too much when you increase the aspect ratio without reducing the rim diameter. Google search for "Tire Size Calculator". There are several like this one https://tiresize.com/comparison/ You can put in the original tire size and compare it with other tire sizes to see what the impact would be. Grab a flashlight and look under the fender (or is it a torch and look under the wing?) The top of the existing tire is already pretty close to the upper steering knuckle and the outer edge is pretty close to the fender when cornering hard under a load. The larger the diameter, the closer they get.

I think I remember reading a post by someone who was running a 245/45r19 but I think a 245/50r19 would likely rub as its almost another 2 inches in diameter. One of the things I did before buying a final set of tires was I went to one of those independent shops that sells used tires and got them to mount a dirt cheap used tire the size I was thinking of and went for a test drive to make sure it was going to fit without rubbing and not look weird or anything before buying a whole set. If you want to try going up a size on the same rim, that's one way to test first.

 

>The problem is that the overall diameter grows too much
That's true, the tire/rim combination increases in diameter and you run into clearance problems and your speedo/odo would be all screwed up (wider tires will rotate fewer times over the same distance as the original wheels, thus messing up the speedo/odo). They may also rub against things particularly when turning.

On the flip side, trying to go to smaller wheels can also run into clearance problems. 16" wheels from older Jaguars probably wouldn't fit because the brakes of modern Jaguars are so much larger. That means that a 16" wheel might not fit over the brake rotors. You have to be careful.
The best advice is to start by seeing how many different sized wheels were offered when the car was new. If the car originally only came with 19" I'd be worried about trying to fit 18"s or especially anything small than that.
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Understeer is when you hit the wall with the front of the car
Oversteer is when you hit the wall with the rear of the car
Horsepower is how fast you hit the wall, and
Torque is how far you take the wall with you

 

I'd already figured out that 18" is likely the smallest you can go on the OP's particular trim level. The XF's with the 4cyl engine, some euro diesels and some earlier years v8s did have a smaller brake rotor. In some markets, base trims came with 17" wheels over 326mm front rotors. However, in 2012 in the US, the v8 Portfolio edition and the base model both got the 355mm front rotors. The base model came with 18" wheels while the Portfolio came with 19" wheels as simply part of the appearance package. According to another thread, there were no Jaguar 17" wheels that would fit over the 355mm caliper but there was speculation that there might be something in the aftermarket. Just looking at it, I highly doubt it tho. The supercharged v8 XF models got even larger 380mm rotors and 20" wheels. If somebody wanted the extra comfort bad enough, the parts would be readily available to switch out to smaller brakes AND smaller wheels. Personally, I think the 18" Velas look much better than the 19" Caravelas anyway.

 

Back to the OP's original question.
He has 245/40R19 tyres and would like to go to higher profile tyres on the same wheels for increased ride comfort.
The maximum feasible change would be to go to 245/50R19 tyres, this would increase the tyre diameter by approx 49 mm and reduce the wheel well gap to the top of the tyre by approx 25 mm. I for one don't see this as a problem as the stock XF wheel gap is fairly generous to start with (maybe not 4x4 huge but close!) and plenty including me have fitted 30 mm drop lowering springs to the front of an XF with zero problems.
The increased diameter would also cause the speedo to under-read by around 7%, but IMHO that is a good thing as the XF speedo over-reads by around that amount anyway so the speedo would end up pretty close to spot on.
The other alternative would be 245/45R19 but I doubt that would result in the increased ride comfort the OP is after.