michaelh, another XK8 conundrum pertains to the tire inflation pressure. All the steel belted radial tires I've ever purchased recommend tire inflation pressure of 35 psi. However, the tire inflation sticker located in the storage caddy between the two front seats states that for 18" wheels, the recommended tire inflation pressure is 26 psi (front) and 28 psi (rear). If the XK8 owner follows the jaguar recommendations, the the tires are underinflated in terms of the tire manufacturer's recommended tire inflation pressure. So, the owner faces a dilemma: Should he follow the tire manufacturer's recommendations for tire inflation pressure? Or, should he hew to Jaguar's recommended inflation pressure per XK8 factory specs?

 

My wife's 2006 XK8 has 19-inch staggered wheels. During the first year of ownership (2012), I experimented with tire pressures in an effort to learn the optimal settings for comfort as well as for treadlife longevity. I eventually came to the conclusion that 32 psi front and 29 psi rear is best and have kept the pressures there ever since. Running much more than 30 psi in the rear leads to a much shorter treadlife....

 

Jon89, your recommended pressures, which you determined empirically, make intuitive sense to me. Why? Because that heavy V8 means most of the weight is on the front tires. So, it seems logical that a higher inflation pressure of the front tires is needed to avoid distorting the shape of the tires caused by weight. In other words, if all four tires have the same inflation pressure, then the front tires will bulge more than the rear tires.

 

The inflation sticker in mine gives two options for the 18" wheels: 28/24 for 'maximum comfort up to 160kph/100mph' and 32/30 for 'normal use'.

I set mine somewhere in the middle at 30/28 as 32/30 is higher than most use here, and the car rarely sees north of 100mph...

The XK is pretty well balanced. Around 51/49 front/rear, or so I've read.

 

On my 2002 XKR with 18” wheels, and Using the Jaguar recommendation for normal use results in excessive tread wear right in the center tread, a dead giveaway for over inflation.

Conversely, using the lower Jaguar recommendation for maximum comfort and speeds under 100 mph is too low, and the cars handling suffers, as well as tread wear all across the tire.

My compromise is to use a midpoint between the two recommended pressures. The result is decent and even tire wear and and best handling I’ve had out of the car in a variety of driving conditions, both at high speeds and lower (sane) speeds.

My numbers are midpoints of 29 psi front and 31 psi rear.

Z

PS there’s more than weight distribution that comes into play when setting tire pressures. Remember, the rear wheels are ones being driven by the engine. That’s why, among other smaller considerations, Jaguar has them getting a smidgen more pressure than the fronts.

Imagine that your tires are a pencil eraser, if the pressure is too low they will “wiggle” more when the power is applied, and thereby wear excessively. Too much air pressure will reduce traction. So it’s a balancing act to find the right midpoint, and often requires some experimentation.

 

michaelh, I'm still trying to get new tires on the car. At "House of Brakes" in San Rafael, I ordered new tires, but when they arrived, they were 17" instead of 18", so I just had them put my old tires back on. Then last week I went to Costco. They didn't have tires that will fit my car in stock, but will try to order them. I decided to go with four (4) identical 245/45 R18 Michelin tires, so that I can rotate them. The car has excessive inside tire wear, so that the tires wear out a bit prematurely. The Costco treadwear warranty doesn't apply if the tires are worn unevenly. I will probably get the wheels aligned after the new tires are on, but I doubt the car will hold an alignment, because of worn bushings.

While I wait for Costco to call to let me know the tires have come, the car is in the paint shop to get the hood, one front fender, and rear top tail light housing repainted. I realize they can't match the paint color exactly, but I'm hoping it's close. (Fingers crossed...)

 

To all who are still following this thread, my sincere thanks for your continued interest and support! I apologize for the long delay in posting an update, but it took weeks and weeks to get new tires on the car.

UPDATE: The body shop matched the paint color so precisely that it is almost impossible to determine which panels were repainted! I'm so pleased! Next, I bought 4 new 245/45 R18 Michelin tires at Costco. That completely solved the vibration problem! No more vibration! Hurray!!!!!! Next, I got a 4-wheel alignment. Now, I'm taking care of some deferred maintenance issues: replace engine coolant, replace brake fluid, etc. I'm still on the fence about whether to replace the transmission fluid & filter. The mileage is 88,170, and the consensus on Jaguar Forums seems to be that one should do it before the odometer hits 100K miles. Hmmm...

 

I wouldn’t put off the transmission service any longer.

Z

 

These ZF 6HP26 transmissions can greatly benefit from a fresh ATF drain-and-fill every 40,000 to 50,000 miles. Keep in mind that the friction modifiers in any flavor of ATF wear out due to time and heat cycles. We did the first one on my wife's 2006 XK8 at 67,000 miles (a pan drop, sleeve replacement, and ATF refresh). We have done three more simple ATF drain-and-fills since then ("simple" means no pan drop, but there is nothing simple about the process as those of you who have done it already know). The car is now approaching 125,000 miles and its ZF continues to perform exactly as designed....

I stayed with ZF Lifeguard 6 ATF for the first two drain-and-fills. I switched to Ford's Mercon SP ATF for the third and fourth drain-and-fills....

 

^^ What they said.

It's due for change on age alone.

 

Jon89, thanks for the advice. By the way, have you replaced the transmission filter on your wife's jaguar yet? If not, why not?

 

michaelh, should I replace the filter along with the ATF? (Yesterday a mechanic told me that in these old transmissions, sometimes the little metal chips in the ATF are what holds the transmission together, and if you change the ATF and filter, the transmission may fail.)

 

If you will study some of the ZF 6HP26 ATF drain-and-fill threads on this forum, you will learn that the pan is also the filter. So yes, when we dropped the pan at 67,000 miles and installed a new pan as part ot the process, the new pan provided the new built-in filter....

 

Jon89, thanks for clarifying that. All the best!

 

I don't think I'd want metal chips floating around in my ATF...
I wonder if he's thinking about a transmission flush, which some opine can dislodge deposits in the trans and cause problems?

Since you don't get all the old fluid out on a drain (some remains in the torque converter), it's not a bad idea to repeat the drain/fill after a couple of hundred miles to further dilute the old stuff. Not as painful on the wallet now if you choose to go with Mercon SP.

Make sure that you can get the fill plug loose before starting the job.