I've been thinking about fitting wider tyres on the rear since I'm due some soon. 285s on the back don't seem much good with the power it's got! Can anyone tell me what my options are? Cheers

 

The launch off the line won't vary that much by going to wider tires. This is because the PSI on the contact patch will decrease as the contact patch increases.

The primary benefit is temperature management in sustained conditions from spreading out the friction from the contact patch over a larger area.

This is also why a 4,000lb car can match a 2,000lb car in a corner. However, tires being equal between the two cars, the 4,000lb car will have its tires give up sooner.

The lighter car also sees the same amount of aerodynamic downforce or lift as a great percentage of its contact patch pressure; resulting in more drastic performance change from aerodynamic advantage.

Mostly what you encounter is that the engine and gear box are up front; so you don't have as much of your total mass for grip to launch with. The best way to get a better launch is to get a car like a Corvette with a rear mounted gearbox, or a 911 with rear mounted engine and gear box. You could also sacrifice cornering by going to a drag racing suspension setup to maximize weight transfer at launch.

You mostly will be going wider for looks, unless doing multiple track laps in a row.

 

I bought a set of wheels from and XKRS and fitted them so the rears are now 295

 

Assuming you're not going to sell your car and buy a Corvette or 911 () I'm afraid there's not much you can do, as PsiFox mentions changing wheels is an option, you could fit wider tyres on the existing rims but it's not advisable and I'm not sure you'd notice any benefit.

What tyres are you currently running and how old are they? It may be that just putting new rubber on may be all you're looking for, my car came with Falkens which struggled, I switched to Pirellis and it still struggles but is much better. If you fit something like the Michelin MPSS I imagine it may make a big difference (like I say depends what you're currently running).

 

When I bought the wheels I went after much review reading for the new Michelin Pilot Sport 4S.
The level of grip both wet and dry is simply amazing. Tyre noise is also much reduced which is rather nice when driving top down.

 

I'm going to go the other way when I change my rear tyres out. Standard XKR rims so I should have 285 rears, however the PO had fitted 295 tyres so the tyre appears to 'bulge' like a balloon beyond the wheel rims.


I intend to go back to standard 285 next time in the hope that a more square sidewall profile that should come with the 285 tyres will help with the rear end wiggle I get when putting the foot down.

 

Well I hope for your sake it works, but I think you'll probably be disappointed, although I like to think having too much power is a nice problem to have!

 

Thanks everyone. I have the Dunlop SP Sportmaxx on and I have been looking at the Michelin Pilot Sport 4S as my rears are due soon (for obvious reasons) if they're as good as some of the reviews say they are then I shouldn't have a problem! Lot of money to spend to find out I have the same problem that's all!

 

If you want more grip, you need a softer tire compound. Try Michelin Pilot Sport Cup 2, PZero Trofeo R, Dunlop Sport Maxx and similar. You will have increased grip but they will wear out faster. No free ride

 

The Pilot Sport 4S is a harder tire than the SP Sportmaxx that is coming off. If you are looking for better traction you want a softer tire. The Pilot 4S will probably wear longer but don't expect better traction.

 

The R-S wheels will take 305s - I have them on my Vortex rims. They are 5mm wider each side, which isn't really noticeable.

 

Having used both I have to disagree. The whole technology of the 4S is new and updated.
reviews have rated it consistently above everything else and traction is most certainly an improvement. It is of course a summer tyre and I'll be using sottozeros in Winter months.
Someone has also tested both the 4S and the Sport Cup 2 and said unless you do a lot of track days you won't notice any difference so buy the 4S.

By the way I'm in no way a Michelin fan boy in fact I haven't used th brand in nearly 30 years. Until now I've always liked either Pirelli or Goodyear Eagles.

 

On the xkr-s, about the widest tire that will fit is the standard PZero ( not the J, jaguar spec ) stock front size 255/35/20 tread width is 10" this is real close to the steering knuckle but it does not rub. Rears you can use 305/30/20 which has a 11.7" tread width. No grip below 60 degrees but ok in warmer dry weather - typical PZero.

 

I realize now that I forgot that the tire compound, I fixated on the physics of contact patch and forgot compound.

Getting a shorter lasting grippier compound can make a big difference. Also replace your tires if you feel the compound has hardened and aged.

 

I have 295/30R20 Pilot Sport 4S on the rear of my XKR, on the standard 9.5" wheel. I wouldn't say they balloon out off the rim.

But they still look kind of narrow, i really want to get a 10.5" rear wheel which should stretch them out a bit further, only an X150 can make a 295 look small LOL

It looks to me that it would be possible to even fit a 325 or 335 on the back, if you had the right width and offset wheel. There's a good 40mm clearance on the inside and you could easily push it out another 10-20mm before the tyres poke out past the wheel arch...

 

Yup, when I was wheel shopping I saw the rear has a *lot* of clearance. The front not as much.

 

Interesting - I have 265'35/20 MPSS on my Vortex 9" fronts, with no rubbing. The 75 has a revised front upright, prototype of the Dynamic Pack & R-S, so I'm surprised there is a difference.

 

Yes, wider tired with a larger contact pattern will grip better. It also IS true that a larger pattern will reduce the weight on each square inch of tire contact, but that's plain science.
(The actual weight of your car can be figured using the size of the tire contact area times the PSI in the tires. Not SUPER accurate with low profile tires as the sidewalls DO bear some weight which mucks up the math, but that's the basic principle)
To easily prove traction increase, do the "EXTREMES" scenarios..... Bicycle tires have a small contact area, put them on a car and you'll have 1000 pounds+ for each... I'll say square inch of contact. That will also dictate a tire PSI of 1000 PSI. It'll give you almost ZERO traction.
Put super-mongo fatties on, lets say two feet wide and three feet diameter. Contact patch say...... two square feet? 1000 pounds divided by 288 Square Inches is about 3.5 PSI. Your traction will be MASSIVE.
Why else would Drag Racers put huge tires on their cars?

 

So they can reduce the pressure and increase the contact area.

 

An important thing to remember is that one 295/30/20 tire will not have the same tread width as another brand. The PZero has a tread width of 11.4". Same size in Potenza S04 measures 10.3". Pilot 4S measures 11.1". Section width will be the same but not tread width. This could explain why some owners can run 265 wide fronts and others can't. Depends on the brand. Having too wide a tire for the rim size makes the car slower around corners, accelerating and braking. Unsprung weight increases and the tire rolls more on the rim ruining handling.
Tire weight, a tire size appropriate for the rim size, tire compound or wear rating, what temperatures the tires will be used in, tread width and design are all important factors in selecting the right tires for your driving conditions and driving style.
Tire rack has lots of good information on their site so you can make a good decision on your next set of tires.