Hey Mike, there's a lot more to look at when buying tires than just the Z rating. That particular rating is really just how fast the tire can spin - there's much more to take into consideration. For me the two biggest things are treadwear and water evacuation capabilities. My all-time favorite tires so far have been the old Toyo T1-S's followed by either T1-R or Continental DW Extremes. Both have decent treadwear rating - around 300 - and great water evacuation which makes them a blast to drive yet also safe enough for wet weather. My current summer tires are the Michelin Pilot Sport Cups which EVERYBODY touts as being the best non-competition (defined here as less than 200 treadwear made for competition) daily driver tires on the planet. they are GREAT in the dry but their water evacuation capabilities simply aren't on par with the Conti's or Toyo's that I've owned.
With all this being said I would feel your best plan is to take into consideration your environment, how long you want the tires to last and seek input and reviews from others that have a similar driving style and personal criteria.
I would just hate to see you buy tires based on one specific rating and not be happy with them because when you really get down to it tires are the most important part of the car because all you get is four tiny contact patches that keep you planted.

 

Just turned over 104656 miles after a 3154 mile trip to Tucson and Houston.

 

My 2006 just turned 17,000 miles. I thought that would be the lowest mileage on here, but the dude in Malta's only got 12,000.
Behind it you can see the '03 X-type that this one replaced with 93,000 miles, as well as my '04 XJR with 41,000 miles.

 

 

That's quite a good mileage. Mile turned 150,000 km (around 93,000 miles) this month.

Your loss of power can have many sources, check the forum for more information but I'd say check the spark plugs and coils first!

 

jaguar x - type 2.5 v6 awd 150k+ im the second owner...but she is showing the age...!! replacing all the suspension and i also need to change the rear sub frame and all the shocks absorbers and all the nuts and bolts are rusted but soon (till friday ) will be ready the headlamps are my biggest nightmare i just bought 4 led BULBS but the high beam on the passanger doesen`t work..the rest is ok ...i will go tomorrow to a scarpyard to get a set of headlamps and try to fix...also car has a ton of bits and bobs to be fixed/corected...rust is the major one..!! i love-it and i hate this car!

 

Mmy 2003 X-Type has 136,478 miles,Needs all new bushings,arms,springs,etc,and the tranny,TC,DT fluids hasnt been changed

 

My 03 X-type has 162700 miles on it. Still going strong.
Have to replace master brake cylinder this weekend. Any advice would be appreciated.
Don

 

As of today 105,411 miles.