I would like to take my XJR to the occasional day at the drag strip.

With that in mind, I have been looking at D.O.T. approved drag tires.

My XJR came stock with 18" x 8" wheels, and 255/40 R18 tires.

First off, let's talk tire widths. Most 255 wide tires have a recommended wheel width of 8.5" - 10". For some reason Jaguar decided to mount these tires on extra-skinny wheels of 8" width. Strange. For 8" wide wheels, they would typically recommend either 235 or 245 wide tires. Anyone have any insight as to why they would do this?

Also, mounting a tire on an extra-skinny wheel will decrease the section width of the tire. A 255/R18 tire is usually measured on a 9" wide wheel. Every 0.5" you reduce the wheel width by, you reduce the section width by about 2/10". This means that if a 255 tire is 10.04" wide (255mm / 25.4) when mounted on a 9" wheel, it will only be 9.64" (245mm) when mounted on an 8" wheel.

So instead of mounting a 245mm tire on an 8.5" wheel, they increased the tire to 255mm and mounted it on an extra-skinny 8" wheel.

This might have something to do with load rating. Jaguar recommends the Pirelli P-Zero tire for the 18" wheels. These have a rating of 99Y. The 99 means each tire can handle 1,709 lbs while the Y means 186mph. This is a rediculously high load rating for such a high-speed tire. This probably has to do with the fact that this car weighs 4,200 lbs WITHOUT PASSENGERS!

Let's say I wanted to go the "normal" route, and mount 245/R18 tires on an 8.5" wheel. I would still want a 99Y tire. I could go with either a 245/40R18 (25.72" diameter) or a 245/45R18 (26.68" diameter) tire.

In the 245/40 category, assuming I want to maintain the Y speed rating, the highest load rating I could find was a few tires with a 97 rating (1,609 lbs), with most being 93 rating (1,433 lbs) or lower.

In the 245/45 category, assuming I want to maintain the Y speed rating, there are several with a load rating of 100 (1,764 lbs). Perhaps this option didn't give the "low-profile" look Jaguar was after (4.34" sidewall vs. 4.01" sidewall) ?

Secondly, let's look at diameter. The the stock tire was approximately 26.03" in diameter. With passenger tires, it's best to stay within 3% of the stock diameter. This means going no larger than 26.81" in diameter.

As far as D.O.T. drag tires, the options I have come up with so far are:

255/45 Category (stock size):245/40 Category:245/45 Category:Out of these options, I personally would not be comfortable running an 88 tire (rated for only 1,235 lbs) when they recommend a 99 tire (rated for 1,709 lbs)!!!

Looks like either the Kumho Ecsta V710 or the Hankook Ventus Z214 is the best option.

From what I've read, it sounds like the Kumho's have slightly better dry traction than the Hankooks (9.5 vs. 9.1 rating), but the Hankooks have better tread life than the Kumho's (7.9 vs. 6.5). The other big difference is that the Hankooks are about $100 cheaper per tire than the Kumho's ($210 vs. $310).

So that's the tire that I think I will try out. Just figured if I put all this information together for myself, I might as well share it with the rest of you too. :-)

-Caleb

 

My XJR had 245/45-18 Pirelli tires on it when I bought it. They didn't look too tall, but IMHO they did look a bit narrow...





I've replaced them with 255/40 Goodyear rubber, but don't have any pics yet (still driving with winter tires here in Finland) for comparison. But they sure do look wider on the rims. One thing to consider is that different manufacturers measure their wheels differently, i.e. Hankook 245 might not have as wide tread as Kumho 245. A bit like treadwear, you can't make comparisons between makes, as there is no standard for the measurement.

 

I have 255/35/18 Goodyear F1 summer tires, which will be installed next week. Basically sounds a bit too low, but almost new ones and bought by the previous owner. If you need good traction, buy MT Street Radial 245/40/18 if you want to run with orginal wheels. If you have another wheels , use higher profile. That gives better results at the strip. Before the street slicks, install LSD. Otherwise you do not get all the advantage.

 

How about installing 10" wide XK rear 18" wheels on the back for more traction? The added width might just help?

 

My original thought was to find a drag tire that would fit the stock wheels, since I've got a few extras now (thanks to Jaguar4s' part out). :-)

However, I wouldn't be opposed to running wider tires at the rear. I could get a set of 275/35R18 Hankook's, and mount them on anything from 9-11" wide wheels. I'm just not sure how much room I can get away with. I guess it also depends on what offset is used. I believe the stock offset is +33mm?

Anyone out there run a wheel wider than the stock 8" on their X308 XJ8 or XJR?

 

The optional Milan were 8.5

 

Are you going to be daily driving the drag tires or are they just for trips to the drag strip? If only for drag strip use, ditch the 18's and get 17's with 275/40R17 tires. Or the smallest diameter wheel that fits over the brakes with widest tire that fits.



You could also get 9" wide Milans.



.

 

My thesis is that your rear end will fail sooner with stickier/wider tires

 

Are you saying that 18" x 9" wheels were an option for the X308 XJR?... or where those only for the XKR?

I took a look at the stock 18"x8" wheels I've got mounted on there right now. There's only about an inch between the inner face of the tire and the suspension components, and there's less than an inch between the outer face of the tire and the fender lip. 18"x9" seems like it would be cutting it VERY close?

And yes, this would be for track use only.

-Caleb

 

I thought the main advantage of a Limited Slip Diff was in corners and road racing? Assuming you've got grippy enough tires, does a LSD provide any benefit?

-Caleb

 

I wrote a long-ish reply here while my XJR was having it's a/c serviced, but the laptop battery decided to die on me...

Anyway, open diff only works when both tires are able to convey exactly the same amount of torque, and on a drag strip that is possible only in theory, or by luck. Or by not using all the power you have. If the engine has enough power to spin one wheel, it will eventually do so

 

I was saying you could get the Milans in 18x9 as well. I'm not sure if the Milans came in 8.5 or 9 wide on the rear of the XJR, but they were 9" on the XKR.
Anyways, fitting 9" wide on the X308 is no problem. 10" wide have been fit with the correct offsets and maybe a little fender lip trimming.
For drag racing purposes, you need the largest tire with the tallest sidewall that will fit in the wheel well, mounted on the smaller diameter wheels that still clear the brakes.
Off course for road racing they would be useless.

 

Which 18x10 wheels were these?

 

Has anyone here put 9.5" or 10" wide wheels on their XJ8 or XJR with a 265mm or 275mm wide tire? If so, I would be curious about what offset was needed, as well as whether or not any lip rolling/trimming was necessary.

Thanks!

-Caleb