They are going to be 18's, but I have a tire choice...
225/40/18

or 245/40/18..

what the hell is the difference!!

 

Great question!
I'm not an expert here... but that's never stopped me from having an opinion!

If I understand correctly, the first number is the width of the tire, represented in MM.
the second number is the "Aspect Ratio", related as a % of the width, and defines the "height" of the sidewall.
In other words: a 225/40 is a 225mm wide tire, with a 90mm tall side-wall. (90mm is a 40% of 225mm)
and a 245/40 will be a 245mm wide tire, with a 98mm tall side-wall.

What size comes as original equipment on your jag? If it's the 205/55/16, then you are looking at a package that is 205mm wide and 631.75mm tall (assuming the 16" wheel has a true height of 406.4mm)
In the case of the 245/40/18, you're looking at 245mm wide, and 653mm tall (assuming the 18" wheel has a true height of 457mm) that's 7/8" taller.
Now, which will fit under the Jaguar? I've no idea. But others here will know. Next question is how does the X-type's speedometer work? Will we be able to adjust for the taller tire?

I've updated this with correct math. (I originally added the sidewall to the wheel only once, when in fact twice would be more correct)

 

Good one McJag..

here is a tire calculator to get you started to see what will work..

http://www.wheelsmaster.com/rt_specs.jsp

225's will give you a 25.1 " tall tire..a 245 will give you a 25.7" tall tire..consider the width too for turning clearences.

 

My car is lowered too.. not sure if that affects anything.
The rims on there right now are the factory jaguar 17" sport ones..

sooo... if I get the 245's the car will make bigger turns?
I dont like the idea of that...

I would think the 245's would look nicer though..

 

and one more thing..
would 18's mess with the acceleration at all?

 

well, anytime you increase the tire diameter, you in effect lower (numerically) the final drive ratio. so yes. it 'could' slow your acceleration. but that difference probably won't be noticeable in this case.