Ever since I replaced my factory 18" "impeller" wheels with 20" low-profile wheels and tires, I have wondered about the correct air pressure (and since the max was 50lbs I usually inflated them to 40-45 or so).

So I did a little bit of research and found some very useful information that I would like to share. I will post the "answer" I found at the end, but in summary here is what I did (refer to the screen grab below):

I found the original recommended inflation specs off of the placard under my arm rest and from that info I looked in the attached pdf (Tire Replacement Manual) to find the corresponding load limits for those size tires (pgs 41 & 42)... I then went to the section for my new tires (pg 44) and looked for those same load limits and I noted the corresponding pressures for those same load limits...




So as you can see from the above table, if I want to have a similar "comfort pressure" on my new tires I need to inflate the fronts to around 31.5 and the rears to around 34.5 or so (and the normal pressure would be 38.5 and 42).




This will make for a long post, but I will now detail the complete answer that I found for determining the appropriate tire pressure when changing tire sizes:

The air in your tires is what carries the weight of your vehicle. Tires are in essence a fancy container for air. Tires have different load carrying capacities at different inflation pressures though and you can't use the same pressures on tires of different sizes. If you installed 20-inch tires and on a vehicle that originally had smaller diameter tires, like 16's or 17's then you are definately going to have to re-think your pressures so its good you asked. But how do you do that?

There is a very scientific but not too complicated way to figure all of this out accurately. Industry standards dictate that when plus sizing you need to match the load carrying capacity of your new tires to that of the tires that originally came on your car at the factory recommended inflation pressures. That means you need to get familiar with tire load and inflation tables.

You didn't say what size tire or what vehicle this is going on so I can't make specific recommendations. I can show you how to do this yourself by example though, so lets pretend for a minute.

Let's say our pretend vehicle came out of the factory with a Passenger-Metric sized P225/60R16 97S tire with a recommended pressure of 32psi front and rear. This equates to a load capacity of 1,521 lbs. To maintain the correct overall diameter we select a tire with the dimensions 245/35R20. Both tires are approximately 26.75 inches in diameter so they maintain the same odometer and speedometer readings and don't mess up ABS and tire pressure monitoring systems.

If we go with a Euro-metric sized Standard Load 245/35R20 91W (91 is the load index of the tire) and consult the load and inflation tables for this size we find that at 32 psi the load capacity is only 1,224 lbs. That isn't nearly enough. In fact the maximum load capacity for this size is only 1,356 lbs achieved at 36 psi - so you shouldn't use this tire at all, but there is another option with the same dimensions.

If we try a Euro-metric Extra Load 245/35R20XL 95W (note the higher load index) and inflate it to 42 psi it has a load capacity of 1,521 lbs - exactly the same as the stock 16-inch tire at 32 psi. So that is the tire size and pressure we need to run.

To figure out the needs of your specific application, first refer to the placard on the drivers door frame of your vehicle to determine the exact size of tire that came on the car (including the load index number, not just the dimensions) and what the recommended pressure is. Then refer to the load and inflation tables that start on page 28 of this document:http://www.tiresafety.com/images/Tire%20... to find out what the load capacity of the original tire is at the recommended pressure.

Determine the dimensions of the 20-inch tire you wish to use or already have along with its load index number. Then refer to the same tables to find the inflation pressure at which the load carrying capacity matches that of the original tires at the factory recommended pressures.

Its that simple.
ASE Certified Automotive Service Advisor with 5 years experience in the tire industry

Hope this was helpful!

 

If you indeed decide to pump your rear tires up to 34.5 psi, be sure to let us know how long they last before the center tread is completely gone....

 

It would appear the load rating matching is the most critical point. Hence a identical load rating would allow pressures to remain fairly constant?

Maybe a thread on tire pressure / tire sizes would be good to try.
example : Ft - 245/45/18 (need load rating next) 31psi

I take interest on this since I moved from 18" to 19" rims / Tire size as well increased from 8" to 8.5" (255/40) front and from 9"- 9.5" rear (275/35)

 

Jon - Nothing personal, but we are not talking about the old bias ply skinny 15" tires from years ago where you could SEE when the tires were over-inflated... I have been running 40-45psi in my tires for the last 2.5 years (those 20" tires in the excel snippet above) with NO detrimental impact on the center of the tires...

Do some research, because a lot of FACTORY SPECS for various cars recommend tire pressures in the high 30 range - from BMW, VW, Mercedes, etc...

Thanks!

 

I appreciate your viewpoint, but the real-world experience of most XK8/XKR owners during the past few years (on this forum, anyway) consistently shows that if you run your rear tires at more than about 29 psi, your center tread quickly disappears regardless of whether you have 17, 18, or 19-inch tires. I would think that 20-inch tires would trend even worse....

How many miles do you currently have on your rear tires?

 

Am running 35 psi on my 20" tires, never had the centre wear any faster.

 

The Jaguar recommended setup for 20" wheels is:

Front 9J x 20" with 255/35 ZR 97Y tyres @ 32psi
Rear 10J x 20" with 285/30 ZR 99Y tyres @ 30psi

I run mine at these settings for 5 years and have not experienced problems with tyre wear, these pressures also add to the ride comfort.

I notice the OP is not running the recommended size or load index tyres with possibly non oem wheels?
So anyone with original equipment 20" wheels and correct tyre sizes should not follow his recommended pressures!!

Also blindside have you had a 4 wheel alignment check for the different sized wheels/tyres ?

 

Stumpy - EXACTLY... but the whole reason for my post was because I (like many others) have aftermarket wheels and tires and wanted to know what the proper pressure SHOULD BE set at...

And NO I have not had a four wheel alignment done... the wheels are fairly close in overall diameter compared to the originals and I notice no impact on the ride (pulling or squealing or weird tire wear). I DO need to get an alignment since I have never had it done, but I need to replace my rear shocks first...

Can someone post a pic of the tire pressure placard under the arm rest so I can see it please (mine only shows 17" and 18" and I want to see the 20" section)...

Thanks!

 

I have been following this thread with great interest, as tire pressure is critical to many areas of operation...ie, handling, tire life, and ride quality.
I would caution those individuals who like very low tire pressures for perceived ride quality that there is a hidden danger in that behavior.
There is a direct correlation between a tires inflation pressure, and it's hydroplane threshold speed. The lower the pressure, the lower the speed at which the tire will hydroplane.
The vehicle manufacturers know this and recommend tires and pressures that are an optimum balance between load capacity, ride quality, handling, performance, and hydroplane resistance (at about 60 mph). I have seen tests where under inflated tires have allowed hydroplaning at speeds 10-15 mph below normal.

 

During the past three years or so, I have consistently run the 255/40/19 rear Yokohama YK580 tires on my wife's 2006 XK8 at 28 to 29 psi. She gets right at 33,000 miles out of her rear tires at this pressure. Her 245/40/19 front Yokohama YK580 tires are consistently run at 32 to 33 psi. She gets nearly 49,000 miles out of her front tires at this pressure. The car has never, ever hydroplaned even while being driven at highway speeds during the heaviest of downpours....

If I bump up her rear tires to just 31 to 32 psi, they are both completely worn out right down the center by 18,000 miles....

A number of other XK8 owners have reported this same experience with their rear tires. I hope they'll see this thread and chime in....

Lessons learned....

 

My 04 XKR is equipped with factory BBS Detroit staggered 20" wheels. Tire sizes Front 255/35 ZR20, Rear 285/35 ZR20. There is a plaque on the underside of the armrest that specifies 32 PSI for those tires. The original Pirelli tires were center worn at 14K miles. I'm now running Continental Extreme Contacts @ 30 PSI and that seems to work. Setting the pressure too high may prematurely wear the tires at the center.


Watch for Potholes this spring ! The low profile tires aren't much of a cushion for the alloy wheels.

 

Jon, That's funny I think she's off doing burnouts when you aren't looking, more often than not Fronts wear due to braking and cornering before the rears..

 

She's never done a burnout in her life. I doubt if she's ever had ANY of her vehicles over 80 mph in more than 46 years of driving....

The suspension geometry of this car allows the front tires to last considerably longer than the rear tires. Add RWD to the equation and it's easy enough for me to accept that fact. We've had the car since early February 2012. It is her daily driver, and she's taken it from 36,000 miles at acquisition to turning 100,000 miles just two days ago. I experimented with the tire pressures throughout 2012 before I figured out that the ideal set-up is 32 psi front / 28 psi rear....

 

I found on my 99xk8 that 30psi in front and 32psi in the rear worked best for me.

 

I am swapping out the wheels and tires on my 2000 XKR with Front 9"x20" on GW 6 Wheels W/ 255/35/20 ZR (97Y) and Rear 10.5"x20" GW 6 Wheels with 295/30/20 (101Y). This was due to a slight bend (front to the left 3/4" after contact) it's on a frame Jig at the moment. Starting pressures? Standard ride height, convertible.
Suggestions?
Wayne

 

The tire pressure sticker on my 2000 XK8 indicates 32/34 PSI (F/R) for 'Normal Driving', and 26/28 PSI (F/R) for "Maximum Comfort". Gus' preferred pressures, 30/32 PSI F/R also work well for me, a good compromise between tire wear and comfort. Gus is the man!

I'm driving on OEM 18" rims (245/45-18 front and 255/45-18 rear). With brand new BF Goodrich g-force COMP-2 A/S, by-the-way; just put on two days ago.

 

The "Maximum Comfort" tire pressure setting listed on the car's sticker of 26/28 PSI F/R is, I forgot to mention, for speeds "up to 100 MPH"it says, so even the comfort setting should be sufficient for many spirited enthusiasts, I might believe.

 

On my 97. I also use the 30 front 32 rear inflation as a compromise from the "comfort" to "normal" recommendation.

 

Most place cards from vehicle manufactures state tire pressure on many factors. Usually max load with standard tire and wheel configuration. Tries, while same in size ( 245 /50ZR-17) quote are often different in actual size. You would note this if you have placed different brand same size tires next to each other. Other factors play heavy on air pressure is RIM width. are they 6", 7" or 10 Inches. Same tire on each rim will change tread angles. Anyway, manufactures are a guide. Experiment is fine and dependent on owner vehicle desire. I do pay attention however, to factory difference front to rear. If so, it is based on rear geometry and handling. I will always stay to the variance front and rear. Everyone is right. You are the expert to your vehicle. Rich

 

Gonna do this right and get a temp reading with a heat gun, after Hiway and after very spirited. That should tell me what I need to know. I raised the load ratin F 97y, R 101y. Filled em with nitrogen and starting at 40 each. 20 miles highway should tell me about "normal" pressures. Next I go to my favorite weekend industrial park and run a course planned to heat front back and both sides the read and try again.