Hi fellow Jag Enthusiasts,
I thought I would share all of this with you although I am not sure how dumb this makes me look by overlooking the obvious first. This post maybe a little long. But here we go.
So I just discovered the wrong size tires are on my XJR6. Just discovered meaning within the last 24 hours. Now I knew the tires that are on there were not the right tires from when I got the car 11 months ago.(First weekend in September 2019 will be a year with my XJR6.) When I got it In September 2018 it came with a small power steering leak in the hoses and a brand new rack in it. Also noticed right away the wrong brand of tires were on the car. General Gmax AS-03, but they had plenty of tread on them so the plan was to just use them a little as is but put the correct tire brand on there before the next winter. Also no triple digit speeds even given the opportunity until the tires are replaced. Granted the ones on there are speed rated at W, but I still was not going to risk that anyway. So I started my search this weekend online @ Tirerack.com. I started by writing down the tire size off the tires on the car and putting that into the search. 235/50 R17. A whole bunch of tire options came up even with a speed rating of W, even in the high performance all season category. That really surprised me truthfully. But I had decided to go with the Continental ExtremeContact DWS 06, but because I was on the web site anyway I compared them to other tires just to see how their performance rates against the competition. I knew I still wanted them though. Just to compare the prices against another website i went to other website, only this website didn't let me search by tire size only brand and model of the car. On that website only one result came up, the Continentals. Then I looked at the details, I realized the tire size i was looking at on the new website was 255/45 ZR17. That was my holy crap moment! The wrong size tires are on my Jag! Quickly confirmed by running down to the car and looking at the tire pressure guide in the center console.
So now I had a question in my mind, why was the wrong width tires on my car from the previous owner? Then I pieced together the answer based on what I had previously discovered on my car. So when I had first changed the power steering hoses and the power steering pump I did notice the steering rack was recently replaced. I recorded the part number on it and looked it up. I discovered the comfort steering rack was in it not the sports rack. I did noticed after that the car still cornered remarkably well on sweeping entrance and exit ramps a lot better than my 1996 Vanden Plas ever did, so I just chalked that up to the sports suspension that the XJR6 comes with. But I guess what really might have happened is the last owner gets his steering rack replaced, notices the steering is not the way it was previously with the replacement rack, so he talks to his shop and tells them the problem, they put on thinner tires as a fix to help the car corner again like it use to. I bet the story goes something like that.
So a width of 235 to 255 gives me 20 less millimeters per tire for a total of 80 millimeters less grip on the road, at least on paper. Think I better get those tires replaced even before fall hits here in New England.
I had actually wanted to replace the shocks first but i think the tires have to get done ASAP. The shocks really are not bad I believe it is just the shock bushings both on the front and rear that have to get changed, but if I am going to do all that work especially on the rear to remove the shocks anyway to replace the bushing i might as well just replace the shocks. Pretty sure I want the Bilstein B6 shocks in there anyway.
Being a car racing fan I know of Koni and discovered they make an adjustable off the car shock for our X300 series of Jags. Those are Koni part numbers 261592 for the front and 301593 for the rear.
I actually sent an email to Koni asking them for more information on these part numbers. Specifically for a recommended starting point on the adjustment. They normally recommend not adjusting them any when you first put them in, only adjusting them after you determine you want more damping then it is set to by default. I pointed out in my email to them that the X300 series of Jaguars that these part numbers are for actually have 2 different suspension setups depending the model you have, I pointed out that there is a comfort suspension like the Vanden Plas and a sports suspension like the XJR, I asked what these shocks are geared toward at the default adjustment and do they have a recommended amount of turns to get from one to the other as a starting point.
When and If I ever get an answer from Koni I will post an answer here in case it helps somebody else. I think I am going with the Bilsteins anyway, I know they make a great product. I was just finding out information about the Koni shocks, I am not really sure about adjustable shocks an a daily driver anyway, maybe if it was a track car

 

No real issue with the tire size if the speed and load rating are correct.

Koni specials (red) are not anymore manufactured for the front according KONI UK customer service based on email two weeks ago so old stock is your option. B4 (green) bilsteins would work with stock ride height but will not work well with (at least did not work for me with HR) lowering springs. B6 (yellow) bilsteins do work quite ok based on couple of tens of kilometers of driving. B4 is not strong enough in rebound damping with the lowering springs.

Just my 2 cents

 

There is a long saga regarding tyre sizes on the XJR6 which you can find elsewhere on the forum. To cut a long story short, there are some members who prefer the narrower tyre as they feel it prevents the car from tramlining, and others, like me, who prefer to stick with what Jaguar intended, as they dont experience tramlining on a well maintained car.
That may explain the narrower tyres, or in could simply be a question of budget/ availability in the hands of the previous owner.

 

Thank you AnttiM.
Actually just received this reply from KONI NA. That was a very fast response.

"Thank you for contacting us here at KONI NA. This is a question that gets asked quite often and while There is no single best adjustment setting for your KONIs because every driver has different preferences for comfort, performance, performance modifications and roads to drive on. For most vehicles, we suggest that new KONIs be installed in the full soft position. (the standard setting right out of the box) to take advantage of the balance of ride comfort and handling designed by the KONI ride development engineers. If the car has performance upgrades (springs, wheel/tire packages, etc.) or the driver wants the car a bit more aggressive, most people find the optimum setting in the 1/2 to one full turn from the full soft range, which in your case since the shock have actual clicks of adjustment so it would be the 1st or 2nd click off of the soft setting. The benefit of the KONI over the B6 is that while they both may be a pane to remove/replace you do have the adjustment option. The B6 if you are not happy with the damping your only option is to replace the entire shock, the KONI simply make and adjustment."

So to summarize above sounds like if you have comfort suspension like a Vanden Plas you would just install the Koni's and not adjust anything. If your putting them in a XJR you would go one or two clicks on the adjustment.
Guess will ask next if they are out of production here in NA too. It could be that they just have a big stock of these in NA as they were just not popular enough here. TireRack and other places have them available if you search by part number.

 

Ok final answer on Koni Shocks availability in NA. here is the email.

"It does appear that KONI Holland has recently discontinued the 26 1592 and 30 1593 shocks. We currently have 4 of the front 26 1592’s left in stock and will the final 4 available. The 30 1593’s we no longer have available."

 

run what you have till worn out. I switched mine to 18's from a 2001 xkr staggered fitment with a 275 rear and 255 front. biggest issue after that was going into those 45mph corners a whole lot faster than before on the way home from work