05 XJR 19" wheels, stock size is 255/40, has anyone tried a size up like; 255/45? Curious what the practical maximum is.

 

Increasing the aspect ratio from 40 to 45 will only make the sidewall of the tire higher. So your car will be higher off the ground and most likely not look right, and the speedometer will read low. The tire will not be any wider.
John

 

Pretty well onto that, and more sidewall is the goal, the question is will it fit without rubbing. Much better to consult with someone who has done it that rather than
trail and error.

 

Hi Eric,

I haven't tried a taller sidewall, but just thinking through the math, the difference in sidewall height between 255/40 and 255/45 is nominally 12.75 mm, so a total of about one inch taller for the upper and lower sidewalls combined. The factory ride height specification is 386mm front and 373mm rear, +/-15mm, so the engineers built in a 30mm margin for height variance as measured from the center of the wheel to the underside of the wheel arch in the fender/wing. If necessary, you could recalibrate the air suspension ride height to adjust for the taller tires. The only remaining question would be whether they would more easily rub on the wheel arches when making sharp turns.

Cheers,

Don

 

I no longer have the air suspension, so that's out, I just had a wheel straightened, & was thinking; if there was just a little more tire between the rim, and the earth. On my X300 i always went a size bigger, and when they were new the R front would scratch just a little on to the lock turns, after a month or so it would wear to fit. That was on 16" wheels, and I never came close to bending a wheel even with some of the stunt jumps on the interstate. That was also on aftermarket wheels, the stockers were so bent I just got rid of them.

 

The coil conversion kits that I've installed seem to ride a little higher than the old (sagging) air suspension, at least right after installation, so maybe you have a little more room for a taller tire?

 

Study that carefully. DonB actually packed a LOT of seriously important meat in that short post, that's worth adding to the decision making. He mentioned the makers have "figured variances"... That means Jaguar ENGINEERED this car to counter turns and stabilize steering motions based on suspension and tire profiles being a certain FACTORY way. When you force a tire ratio to change, the tire's new weight, new treadwidth, forces the car's computering to make tiny vectoring or handling mis-assumptions, and the ride "feels" different. Not unsafe or anything, just DIFFERENT. IF it's only a taller sidewall "look" you're after, and you have no objection to the way the engineers have designed the driving feel, raising up heights that much and offsetting suspension amounts MAY cause a trade-off in the driving experience that you ultimately decide you don't LIKE.

Can I suggest choosing a tire that's got SLIGHTLY less height increase, but has a SQUARE shape shoulder, OR a plainer looking sidewall design. Square shoulder tires make the tire APPEAR to be bigger in the wheel well than others, even when the tire SIZE is exactly the same. And too much tire decoration on the sidewall tends to detract from what a tire is supposed to look like: big, full and rubbery! You can get away with PHYSICALLY increasing your tire half the height you were thinking of (1/2 inches?), and accomplish the rest through ILLUSIONS for making a tire look a size larger.

Unless getting the CAR itself to be taller off the ground is what you're after too...

 

Looks have absolutely nothing to do with it, it is all about saving the wheels, though an inch taller, and it might not high center going into my favorite pizza place.
Maybe better to find another pizza place, but theirs is pretty good.

 

If you hit something hard enough to bend the rim, another 12mm of sidewall is not going to save it... I don't think another 100mm of sidewall would save it.

That said, the folks that do 20-series (and lower) profile tires on their 40-inch rims deserve whatever happens to them!

 

What I find isn't disabling smackaroo rim damage, but instead tiny flat spots that after a few you get a slight vibration @ some speeds. It also causes tires to wear unevenly. Not hard to fix, any competent rim shop can beat them back into shape, but there, another inch of sidewall could save the day.

 

Eric,
Be reassured, this is a very good option.
I did exactly that. In fact the PO from whom I bought the car had done it, exactly to avoid bending cracking rims because of potholes.
More exactly I went from 35 to 40 because my Portfolio has 20" rims (so 255/40 R20 tyres).
I drove 60.000 kms sinc then without a single issue (no bend, no crack), even though I sometimes encountered serious potholes at highway speeds, no rubbing even at full lock, no looseness in handling or steering and probably more comfortable.
Regarding the speedo reading, actually it's much better with this "upgrade" since now it is ...exact! (confirmed with GPS readings, while it was certainly optimistic before)
The stance also looks fine to me, even better and more meaty as the wheel arches are better filled.
So a definitive improvement

 

Thank you for that, I was hoping someone had pioneered this. nice ride by the way.

 

Sorry I can't speak to your particular model, but I can offer one suggestion because I understand. If you're from Texas, asphalt has absolutely nothing to do with paving a smooth surface to drive on, rather its only purpose is to keep the dust down vs just driving our trucks on dirt roads. In addition to a bunch of suspension modifications (that I posted about long ago in another section), I'm running plus sized tires on the XF.

However, for your interest, what I did for sort of test engineering was I went to one of those tire stores on the poor side of town and had them mount a "used" tire the size I was planning to run and I put it on each of the four corners in turn. I can't remember what it cost, but I know it was something less than $50. First I checked all the dimensions on the lift. Then when I was pretty sure it was going to be OK, I drove it around the neighborhood looking for turns and inclines that would max out the suspension travel. I tried them slowly at first and increased speed, listening for any rubbing noises, thus ensuring that there would never be a circumstance that the tires would make contact. I had to call around a bunch of such tire stores to find one with the right size tire in stock, but maybe if you could find a cheap used tire the size you are considering, you could try it that way before ordering a whole set of new ones.

 

Good strategy. We have a wealth of used tire shops locally, so that is totally doable. I have 'make do' tire on the spare wheel, if I was to buy one for the testing, it could be retained on the spare.

 

Hi Serge,
That looks great. My car had 245/40-20 tires (tyres) when I bought it. They needed replacement, so I got the stock size 255/35-20 which work just fine. Here in New Zealand, Jaguar considers this a 'Rough Road' market, which is sometimes the case. Motorways are limited to cities and suburbs. Open country is mostly single lane opposing traffic roads, what the British call a 'B' road. I haven't had any damage to my wheels, as I think the 'CATS' suspension helps reduce the impacts.
I do consider that with the 9 inch rims (I have Callistos too) the 255 width is a little narrow. The sidewalls 'tuck under' to the narrower tread. For my next set of tyres (tires?) I will try 265/35-20. I think that they will look just right, as someone posted pictures here showing them. I think the speedometer error will be much reduced too, as it over-reads by about 7Km/h at 100Km/h.

Pete M

 

Hi, Has anyone tried 255 45 R19 instead of 255 40 R19 on a 2008 Vanden Plas? I live in Staten Island and the potholes and some roads make me wish I had higher sidewalls for a more comfortable ride. I see in the discussion thread that a gentleman has tried 45 instead of 40 for his 20" portfolio rims but mine is a VDP and thus has 19" stock rims.

I understand that it might offset the speedometer and odometer but that difference is slight and can be lived with.
I also understand that the car will be an inch taller that i do not mind
My main concern is whether the 255 45 R19 tires rub against the car during turns and/or bumps?
Will they be harmful for the suspension?

Thanks!

 

IMHO there should be no issue.
Equivalent to 255/40 R20. look my post just above

 

Thanks!

 

Thanks for the suggestions. I'm not terribly concerned about any loss of brake performance. While 19 and 18 inch wheels have an appearance appeal for some, where I live and work in Rochester NY they tend to attract hookers and people looking to buy drugs when I come to a stop for red lights at intersections. Fairly small road surface non uniformities (common in small towns and big cities) easily bend large rims with the accompanying short tire sidewall. Nascar has been driving in excess or 200 mph on 15 inch wheels, and formula 1 racing on real city streets run 13 inch rims. It seems they don't have any problems with repeated hard braking. The fundamental physics of Un sprung mass, rotating mass, and moment of inertia, are draw backs to the crazy rim sizes. My 1999 XKR ran 13.4 in the quarter with the factory 18inch wheel tire (54 pounds each) combination. The same car ran 12.9 with factory 16inch wheels and tires (42 pounds each) from an XJ8. Outside tire diameter was the same in both cases. Hence if the rotating mass is reduced the need for larger diameter disk brakes is reduced. Comfort suspension or not in my Super V8, the ride over average city streets is better absorbed with a taller tire side wall. Additionally a taller tire sidewall will conform to maintain contact with non uniform road surfaces - simply put the tire patch are contacting the road is compromised on a short side wall tire. Enough about fundament physics, Formula 1 Racing and Police Cars don't use 18 inch wheels. I'll save the 19 inch ghetto wheels on my super V8 in case I decide to sell it. Meanwhile I'm on mission to install 17's with 2004 XJ8 brakes on my 2006 Super V8.

 

I'm rather surprised no one has already suggested this, but, use this calculator/visualizer to see exactly what changes in size, aspect ratio, and diameter will do to both fit and ride height. You'd be amazed what works, within the limitations of clearing brake calipers.
https://www.willtheyfit.com/