Whilst previous topics have discussed in detail fitting of 235/50R17 tyres to the XJR in an effort to reduce tramlining I do wonder if anyone out there has fitted 225/55R17 tyres. The reason being that here in Australia at least, this size is more keenly priced due to the fact that they are fitted to many more makes & models. The 235/50R17 is notably more expensive because it is a far less popular size. The same may exist in the US also.
Whilst the 225 does not match up to the original 255 size as good as the 235, I do note that Doug Dwyer fitted 225/60R16 to his XJR with great success. The 225/55R17 & 225/60R16 are similiar in dimensions.
I do not drive like a formula one racer so I am more interested in safer driving (no tramlining or propensity to aquaplaning) with a comfortable ride.

John Herbert
(1996 XJR)

 

I say "go for it"

When I was running 225/60x16 tires there was no sacrifice in normal daily driving. There was a *slight* loss in steering crispness versus the OEM size but you'd have to drive very aggressively to notice any other difference. I suspect the same with 225/55x17....although there are obviously other variables than just size.

Do watch your load ratings and tire pressures, though.

Later, when I went to 235/50x17, I had to drop down to a 96 load rating (versus original 98)....which is removing some of the built-in margin. You wouldn't want to compound that by running low pressure

Cheers
DD

 

Thanks Doug, yes the particular tyre I am looking at is a Bridgestone Potenza, well suited to vehicles of this type.
I had looked at load ratings & it has 97 load rating (730kg) & W speed rating.
I too had came to the conclusion that one would not want to perhaps go below 96 load rating.
Just as a matter of interest the same tyre sells here for $225 in the 225/55R17 size & $349 as a 235/50R17.
It seems obvious polularity of fitment has a big impact on price, just like parts pricing.

John Herbert

 

At the risk of being the perennial killjoy on this subject, could I just point out that fitting narrower tyres materially reduces the contact patch with the road surface. Never a brilliant idea, and in the case of a car of this size and performance, really not very clever. The time when you are most likely to drive like a formula one driver is when you stomp on the brakes in an emergency, at which point, the extra grip of the manufacturer specified tyre size will be at something of a premium.

 

No worries.

I put my XJR thru the paces with 225/60x16, 235/50x17, and 255/45x17 tires and I can assure you that in real world driving the brakes work perfectly well with all 3 sizes....even in severe stopping.

Besides, there's much more to 'grip' then just the contact patch. There are SO many variables in tire design that it would be entirely plausible to find any number of modern tires with better grip than the original equipment size and spec.

Jaguar deemed 225/60x16 as safe enough to make it the standard issue size on the X300s. The other sizes (for the Sport and XJR) were upgrades.

IMO the tramlining experienced with the Jaguar specified tire size is much more troubling, safety-wise, than a smaller contact patch. To be 'in with the in crowd' Jaguar went to a wide, low-profile tire without really tuning the suspension properly to compensate for the negative effects. Even the road test magazines made note of the disturbing characteristic when the car was introduced.

Nowadays we have more and better choices than Jaguar had in 1993-94. We can achieve overall better result.

Cheers
DD

 

Wider tire does not necessarily mean better in terms of traction and safety. Friction is determined by a host of factors: tire width, rubber quality, good brakes (fluid, hoses, master brake cylinder, calipers, pads, etc.). I have also learned from practical experience driving other cars that narrower tires help prevent tramlining, as they do not place as much strain on the suspension arms than wider tires, and less friction means better fuel economy and quieter drive. However, 225/55/17 may look a bit tall on the XJR siblings. So there is somehow some trade-off going for narrower or wider tires. The majority seems to go for the looks, thus wider tires and lower ride. The long-term cost may not be very friendly if there is tramlining, and one would have to forbear the results until the next tire change.

 

Until you get pretty far away from a reasonable size for
a given weight, the contact patch area does not change
for a given weight and inflation. This is not well known
amongst the wider must be cooler crowd. But, it is
fact borne out by not too complicated arithmetic.

A narrower tire will have a narrower contact patch of
the same area. This is an advantage in wet conditions
as a long narrow contact patch performs better in those
circumstances.

Cornering may be less crisp, but ultimate cornering force
is competitive. Holding the vehicle at the limits is also
quite forgiving.

Some experience with pushing big ole American sedans
of the '70's around corners faster than usually thought
possible helps in being comfortable with the above
assertions.

 

Gentlemen,
Whilst I fully respect your decisions to fit non standard narrower tyres,
I do feel that we are now in danger of straying away from reality.
If everything stays the same with narrower tyres, why has the trend over the last 20 or so years been to fit wider tyres? Why, when they wanted to slow cars down did Formula 1 specify narrower tyres? Why, now that they want them to go faster again are they changing to wider tyres? In anticipation of the suggestion that it is all part of some global conspiracy by car makers and tyre companies, ask yourselves this: why do the thinner tyres with taller sidewalls alleviate what you call tramlining? Because they are less directly attached to the road surface..... I wonder how that happens.....
As I mentioned in a previous thread on this topic, I had my current X300 XJR when it was new. It didnt tramline then, and it doesnt tramline now. It does have quite an "immediate" response to the helm, which although commonplace now, was almost exclusively the preserve of BMW at the time it was launched. It seems pretty clear this was exactly what Jaguar had in mind when they fitted the largest diameter wheels they had ever fitted to a production car, with tyres in a unique size to complement them. By all means fit narrower tyres if you prefer, but lets not pretend there isnt a trade off. Most of us may indeed never encounter a situation where it makes a difference, but for my part I prefer to know that should I need it, the tyres are as they were intended to be.

 

Right!




Nobody here has said or suggested that there isn't a trade-off.


Cheers
DD

 

There are certainly some trade off using tires wider or narrower than factory specs.
I found the following article from Car-and-Driver very useful in explaining the effects of using wider tires:
Effects of Upsized Wheels and Tires Tested - Tech Dept. - Car and Driver

 

streets are not F1, and posing a question based on F1 as a rebuttal
is not really an addition to knowledge for the reader if you do not
then answer the question. otherwise it is a distraction.

i could have pointed out that the traction tires used in drag racing
are likely to be a higher aspect ratio. the longer contact patch is
superior for straight line acceleration. but, the OP is not a drag
racer either.


i said contact patch and ultimate cornering power are in fact equal. i did not mention transient response except in passing when referring to a higher aspect ratio being
more forgiving. it is transient response that you are likely associating with steering
being more "immediate" at the helm.

BTW, walls of text are extremely difficult to read. Carriage returns are free.

 

So, translating all of that into plainspeak, I, Jaguar, the entire motor and motorsport industries are correct?

 

Correct in what respect?

When it come to selecting tire there isn't just a single correct choice.

I agree that the race car comparisons are not particularly on-point in a conversation about street driven cars but even the racers use a variety different tires...on the same car.... depending on variables. But it isn't practical for ordinary people to own several sets of tires...so we analyze our requirements and choose accordingly. There is inevitably some sort of compromise. For example, those wanting to stay with the P-Zero Pirellis get with a 'summer' tire with notoriously low tread life. But 20+ years ago Jaguar didn't the wide variety of choices that we have now....including size choices. Jaguar made the best compromise they could at the time....but there's no reason for us to be forever shackled to that decision.

If I find something that better suits my needs, that's what I'll choose....even if it deviates from original spec. For example, I drive hundreds of miles weekly on the interstate highways and in my neck o' the woods we get tons of rain ....so a tire with exceptional wet traction and hydro-planning resistance is a very big safety consideration which outweighs any real or perceived advantage of a adhering to OEM size specs.

If I was purely a dry-weather driver concerned with absolute maximum dry pavement adhesion in cornering and braking I can easily find a tire to suit that need....but it would almost certainly be much less than ideal, possibly unsafe, on wet roads....even in the original 255/45x7 size. Not to mention that such tires typically have short lifespans

(I'll ad that my original motivation for going to the 225/60x16 tires was to get a tire that offered something resembling normal tread life. At the time all the 255/45x17 tires I could find were low-life tires....and wore out after 11000-13000 miles. For me that meant replacing tires every 11 months. That get old in a hurry. The fact that tramlining was eliminated was just a bonus that I was happy to get!)

If you think the only way to be safe is to stick with the factory designated size that's certainly your prerogative but I respectfully suggests that you're ignoring many other variables and considerations

Cheers
DD

 

So can I presume that CountyJag has not read the Car & Driver article, plus the fact that other similar articles are available should one wish to adopt an open mind on this issue.
As stated originally I am seeking a good balance of handling, ride & comfort with a leaning more towards ride & comfort not to mention wet road safety.
Read a few articles on wet road grip with wide or narrow tyres.
Jaguar with this model were obviously seeking ultimate image, handling/response etc. with other aspects becoming secondary & in this respect yes they were on the money, however this does not say we cannot down the line change this objective whilst remaining within normal safety requirements.
The XJ Sport & lesser models are all fitted with 225 tyres as standard. Also interesting to check out the many "high performance" models of other makes actually fitted with this size tyre.
Interesting to note that most new models of majority of makes these days come with larger wheel/tyre combinations either optional or on the higher model lines. Most current road test results indicate though that the lower models (with the smaller sizes) perform better overall on test especially with road testers often commenting that the bottom of the range has the best overall road performance. In the "real world" we don't spend too much of our time on Formula one circuits.
Here in Australia we have a mix of motorways, not many but generally smooth with a large percentage of very average rural roads, plenty of bends yes at times but not the smoothest with bad camber & all sorts of irregularities & challenges.

John Herbert

 

Notwithstanding all points made above by various members, I do agree with countyjag that, all things being equal, wider tires brake better. This point was also borne out in the Car-and-Driver article. The wider tires performed better in terms of braking distance. However, if and when we talk about overall performance in normal use, wider tires do not have much advantage. I am convinced but I still run 215 45 17 and 225 45 17 on my W124 just for the look. I could have gone for wider tires like 235 and 245 or even 18" wheels for a sexier look, but I have retrained myself by striking a balance between look and running costs (fuel, tires, etc.). I myself for one would avoid tramlining at all costs.

 

Guys,
I have been away for a while, so forgive me for not responding sooner.
As ever, the tyre size/ tramlining debate seems to stir up controversy, but on reading the posts again, it seems that there is much that we agree on.
In no particular order;
-We are free to fit the tyres we prefer;
-Narrower tyres are cheaper, more plentiful, and make the car less "immediate" around the front end, and hence less susceptible to what is often referred to as tramlining;
-In normal/sensible driving, the cornering benefits of the wider tyre are marginal, verging on imperceptible.

Whilst some contributors might be content to let the case rest there, it seems to me that there are some important points being overlooked, none more critical than the matter of braking distances.

In the real world, the single scenario where we are likely to seek to exploit the largest proportion of our vehicles performance envelope is in an emergency stop situation. However sensibly we might drive, the possibility is always out there that we will need to stop in the minimum possible distance to avoid an accident. I haven't had to do it in years, and here's hoping I never have to do it again, but my approach at all times to maintenance of the vehicle is to ensure that everything to do with stopping is at 100%, 100% of the time. Tyres are obviously a key component in this, and so maximising the stopping performance of my tyres is a priority.

A Car and Driver article has been put forward as an authoritative source, and I have read it with interest. I am bound to say that it is interesting, but far from a comprehensive analysis, and is quite insubstantial when compared with, say, the annual EVO magazine tyre test. I am not clear on its precise relevance to this debate, as it does not compare different widths of tyre on the same diameter of wheel, but guess what, it does show that braking distances go up as tyre widths go down. Interestingly, it does also make the point that the higher speed rated tyres are made of "grippier" compound, and the current XJR debate seems to overlook the fact that none of the narrower tyres are Z rated, as specified by Jaguar.

A final point, various references have been made to the fact that Jaguar specify narrower tyres for the normal and sport models, so they must be ok for the XJR. Leaving aside the XJRs different suspension settings and ride height, and ignoring its 30% more power and torque, it is some 75 kilos heavier (c5%) than the Sport, itself no lightweight. The needs of the Sport are not the same, and wider tyres with a higher speed rating would not seem an inappropriate or unnecessary response in specifying tyres for the XJR

I have little doubt that the various protagonists in this long running debate (myself included) are as unlikely to change their views as they are their tyres, and that is fine with me. For the benefit of any newcomers to the discussion I hope the facts are helpful to their consideration of how to specify tyres for their XJR.

 

We all choose our own poisons and priorities.

In my real world there are numerous considerations to be balanced, not just one. In this respect, yours is certainly the easier position: a single focus and no variance from a single existing specification.




let's look at the X300 range.

At one end we have the 3.2 'poverty spec' car. It's the lightest of the bunch and has the least power/speed/acceleration. The standard issue tire was the 225/60x16.

At the other end we have the V12 car, the very heaviest of the bunch and with power/speed/acceleration virtually identical to the XJR. The standard issue tire was the 225/60x16.

From this I think we can conclude that Jaguar deemed the 225/60x16 to be fundamentally adequate and safe for a wide range of applications. And, although perhaps not optimal for all drivers in all circumstances at all times, there is nothing fundamentally unsafe about using them on an XJR.


Fair enough !

I have a question for you:

Presumably your XJR has had the tires replaced and you went with the original size. If so, and since emergency braking your prime consideration, are you *sure* that the tires you selected offer maximum emergency braking characteristics versus the many other choices out there? Besides size, what other considerations, if any, went into the decision?

Cheers
DD

 

I think it is fair for countyjag to point out that wider tyres have a shorter stopping distance than narrower tyres (when all other factors being equal), and this should hold true whether in an emergency. I also want to add, however, that, from my own experience driving in Hong Kong (more bends and tight corners around city parkades), wider tyres may also be subject to more uneven wear on the inside of the front tyres.

I think when the XJR6 was introduced, 255/45/17 and P7 was the preferred size and make for the car. Since then many more tyres have become available, and the learning curve has evolved due to more choices becoming available and problems reported in relation to the 255's. I have seen many happy with 245/45/17, including such reasons as accuracy of the actual speed with the speedometer reading.

 

"All things being equal" being the key phrase

I agree.

Cheers
DD

 

As a data point, I have a Daimler DS420 limousine, and the final tire size Jaguar specified on it was 235/70-15. It weighs a full ton more than an XJR (kerb weight ~6,000 lbs) yet that was considered sufficient in 1992.

From a stopping point of view, the XJR is "over tired" in comparison.

I do think a fair comparison is the XJ12/Double Six that have the standard X300 225/60-16 and that seems fine. They also have the same top speed rating as the XJR of 155 mph.