2011 XK - My new rims come in next week - I'm replacing the OEM 20" staggered rims. Staying with the 20" diameter but doing 10.5" in the rear instead of the 9.5". Any recommendations on tire size? I'm sure I can stretch the current tire too but want options.

Thanks!
Matt

 

I've been doing a LOT of measuring of tires and wheels lately as I'm seriously considering doing what you are, but I don't want to get custom wheels only to have them not fit.

This is the results I've come up with so far.....
2010+ XK/R. Front offset is 49, rear is 46.

Rear wheels on Front with <5mm spacer
Rear = 10.5 , 2-1/2 offset , 7-1/2 backspace

SIZE UTQG MAX.
LOAD Max. Inflation Pressure Tread Depth Weight Rim Width Rim Width Sect. Width Tread Width Diam. Revs. Per Mile Country of Origin*
305/30R20 - (103Y) XL
240 A A 1,929 lbs 50 psi 8/32" 33 lbs 9-10.5" 11" 12.3" 11.3" 27.2" 764 CZ


SIZE UTQG MAX.
LOAD Max. Inflation Pressure Tread Depth Weight Rim Width Rim Width Sect. Width Tread Width Diam. Revs. Per Mile Country of Origin*
315/25ZR20 - 99Y RF
280 A A 1,709 lbs 50 psi ?/32" 31 lbs 11-12" 11.5" 12.6" 11.6" 26.3" 789 JP


SIZE UTQG MAX.
LOAD Max. Inflation Pressure Tread Depth Weight Rim Width Rim Width Sect. Width Tread Width Diam. Revs. Per Mile Country of Origin*
305/25ZR20 - 97Y RF
280 A A 1,609 lbs 50 psi ?/32" 29 lbs 10.5-11.5" 11" 12.2" 11.2" 26" 799 JP


SIZE UTQG MAX.
LOAD Max. Inflation Pressure Tread Depth Weight Rim Width Rim Width Sect. Width Tread Width Diam. Revs. Per Mile Country of Origin*
285/30ZR20 - 99W XL
560 AA A 1,709 lbs 51 psi 10/32" 27 lbs 9.5-10.5" 10" 11.4" 9.9" 26.7" 778 CZ

Now I hope that this formatting (from Notepad) doesn't mess up...
Anyway, I pretty much have chosen to go with the NITTO 315/25 ZR/20 tires. It'll be really close (and 10.5 is the absolutely narrowest rim that will hold them) but they should work.


EDIT:
OF COURSE the formatting messed up. Maybe I'll take a screenshot of my text document...

HERE:

 

I have 295/30/20 on 9.5 on the rear of my xj. So im sure you can do a 305/30/20 on 10.5

 

 

As a side issue can the dealer or someone with the correct program capability change the speedo to the different tire size or do you guys just run with the speedo error.

 

the instrument pack programming only pertains to switch wheel sizes not tire sizes staying with the same wheel diameter. Also they are close enough that you are really not going to notice a differnce in the speedo. I can run a 275/35/20 and a 285/30/20, or a 295/30/20 as they are all almost the same on diameter. The 295 is a bit wider. You'll see more of a difference using 2 different brand tires of the same size. I have rolled 2 tires and wheel together of the same size but different brands and they can be vastly different in diameters. Why it is always recommended to stay with the same brand n each axle. But yet there are customers that either dont care, or dont know.
my gps still shows accurate speedo

 

Awesome! After reading a number of posts and threads over the last month and talking to the shop - I went with 20x10.5 +33 for the rears. From what I read, rubbing occurred on the rears around +15-20mm offsets I believe and needed another ~15mm. The +33 was also what the shop recommended for the car.

I've got some 285/30/20's for the rear right now so sounds like they'll work out! I'll get some pics up once the new rims and wheels are on.

Thanks all!

 

I've used this link in the past.
https://www.willtheyfit.com/
Very useful and shows a nice little diagram of the new/old wheel and a brief description of how much it'll change

 

that website is pretty slick. plugged in the numbers - looks like the +33 offset for the rear 10.5 is perfect.

 

Wish I'd a seen that site before my dubious calculations.
Thanks for the link.

 

Since I evidently mucked-up my prior calculations, the website above showed me this, which is better for what I want.... wide as I can get and out approximately the 15mm suggested. What I did figure correctly is that, at least on MY car, there is a TON of room between the inside of the Stock-size wheel/tire before interference occurs on the liner.... like just over 40mm. That's Forever.

 

A while back I climbed under my XKR and took some measurements of the free space.

With a 295/30R20 on the standard 9.5" wheel there was a heap of free space on either side.

With an appropriate width and offset wheel I reckon you could get a 335 or 345 in there without rubbing.



Just remember that the 4.2L cars have a different rear axle to the 5.0L, the ET is different on the rear wheels.

I know all the cool kids want the wheels to poke or be hella flush or whatever they call it, but that is where the rubbing problem comes in on the outer wheel arch.

Keep in mind the XKR-S and XKR-S GT already have 10.5" width rears, so that is no problem, just take the dimensions of the OEM wheel and you're safe.

The Vortex wheel option on the rear of the 5.0L is 20x10.5J+52, for a 4.2L the same wheel design was 20x10.5J+31 (that was the wheel on the 4.2L XKR-S)

On a 5.0L if you have a custom 20x10.5J+33 it's poking out 19mm further than the stock wheel (like adding a 19mm spacer to a stock wheel) so you are getting closer to rubbing the outer wheel arch, be careful...

 

Going to 325/25 opens up a couple more tire choices, as TOYO is the only tire I found at 315/25...
Anyone ever heard of Lionhart tires? New to me. They seem super-cheap, which may mean they ARE super 'cheap'.

 

I guess we'll see - 10.5 +33 was what the performance shop recommended for the 5.0L XK. I'll let you guys know.

 

Even the XKR-S GT on 10.5" rears only had 305/30/20's on the rears, so common sense denotes that a lesser powered car shouldn't need to go wider. It will just be the choice of rubber!