Hi guys
I have to change my tires and planning to do it tommorow.
The current size is 245/50/18 but that size is very hard to find it so I have two choices. I can put 255/40/18 or 245/40/18. I don't know which one to buy. I have seen people using 19" and 20" so will this change of size be an issue? Please be kind enough to give ur expert opinion on this. Thanks.

 

I'm certainly no expert, but I can tell you the size difference. The 255 is 10mm wider tread than the 245. The profile 40 is 40% of the tread width. 10 mm is 1 cm. that's about 2/5's of an inch. The tire height of the 255 then would be an addidional 40% of 2/5". I would go for the 255. Speedo error will be near nil.

 

Hi thanks for the reply. Will it handicap the car handling characteristics? Will I do any damage to the car? Car ride height going to change.... So any other things I should be worried? Any info on this would be much appreciated....

 

Well the height will be unnoticable. The ride may be a bit stiffer as you are reducing the sidewall height from 50% the cross section to 40%. However, many people including myself are running on 30's and do fine.

 

What was the original size came in your car n what is the size you use right now? Thanks

 

the 245/50-18 has a tread width of 8.7" and a diameter of 27.6"...the 255/40 has tread width of 9.8" and diameter of 25.9"...the 245/40 has a tread width of 9.4" and a diameter of 25.7" You can take your choice.

 

Jaguar dealers have software to adjust the wheel size to the car, so the speedo reads correctly, but I'm not sure if tyre aspect ratio can be entered. Apart from that, a lower profile tyre will make the ride harsher. As for ride height, this can also be adjusted using software in JTIS.

 

@Oldmots. I am not 100% familiar with how the tire companies choose to designate their tires, but from strict math I think your numbers are a little off. Correct me if the tire companies fudge a little from strict math.

245mm = 9.65" width
255mm = 10.04" width

Aspect Ratio of 245/50 = 4.82" Sidewall
Aspect Ratio of 245/40 = 3.86" Sidewall
Aspect Ration of 255/40 = 4.02" Sidewall

Therefore, from a total diameter standpoint the three choices would stack up as follows.

245/50 = 18" ID + 4.82" Sidewall + 4.82" Sidewall = 27.64"
245/40 = 18" ID + 3.86" Sidewall + 3.86" Sidewall = 25.72"
255/40 = 18" ID + 4.02" Sidewall + 4.02" Sidewall = 26.04"

From a width standpoint the extra 10mm or .4" I don't think will make much of a difference.

From a diameter standpoint, the closest would be the 255/40 18, but it is still 1.6" smaller.

From a circumfrance standpoint (speedometer affect) the difference between those two option would be:

245/50 = 86.83"
255/40 = 81.08"

That is a percentage difference of 6.6%. At 60 mph the speedometer would be off by 4mph. You would be going 4mph slower than what the speedometer says.

 

This is the size I use and the tire. P235/50 ZR 18 and have had no problems. This appears to me to be the OEM size. This may be more available.

Pirelli P Zero Nero All Season(Ultra High Performance All-Season)
Sidewall Style: Blackwall


Click thumbnails to view above. Most approximate photos shown View Full Screen Photo.


2004 Jaguar XJ8 Vanden

• Size: 235/50ZR18
  Sidewall Style: Blackwall
  Serv. Desc: 97W Load Index 97 = 1609lbs (730kg) per tire
  Speed Rating “W” = 168mph (270kph)
  UTQG: Treadwear: 400
  Traction: AA
  Temperature: A400 AA A
• Price: $144.00 (each)
  Estimated Availability: In Stock
  Estimated Arrival: Get Estimate Pending

 

Going smaller than spec is worse than going larger. Consider this: As tires wear, they get smaller so a smaller tire is only going to get smaller and a larger tire is going to wear into spec, IMHO the lesser of the two "evils".

 

No-ones ever seen this? Tire size calculator ???

BTW the factory spec for the X350 is;



If you had 245/50/18 it was already the wrong size!

As mentioned by Ed, 235/50/18 is the factory size...

 

OK, assuming you have not already bought new wheels............

My XJ6 3 litre SPort has 235/50/18s on. This is 10 mm less thant 245 and barely noticeable I would think, in fact I think my size is the factory fitting. You should be able to get these easily.

 

What did you buy? If I get stuck for tires in Sri Lanka I'll know what to do.

 

Thanks guys. Ended up buying 255/40/18.... Ed, are you from Sri lanka?