I would like suggestions for the best tire pressure for comfort and/or mpg. Thanks.

 

Hi Wolfy,

On our '04 XJR with 19 inch rims, I run the fronts at 30 psi and rears at 32 psi. Jaguar recommends 28-32 front and 30-34 rear. I arrived at my preference by trial and error, after testing the high and low extremes and the middle pressures. I decided the middle ones offered the best compromise of crisp handling and smooth ride, as well as protection of the rims from hard bumps.

On our '93 Vanden Plas with 16 inch X300 rims, I run 33 psi front and 36 psi rear, also arrived at through trial and error.

I'll be interested to read replies from others.

Cheers,

Don

 

By far, the greatest reason for tire tread failure is not road hazard, but under-inflation. Understand that tire and wheel combinations can affect this as well, but generally I run mine at 34 psi front and rear. For me, it yields the best MPG and tread wear characteristics. If you want a slightly softer ride, drop the front by 2 psi.

 

I have all four at 38 PSI. They show on tyre at anything between 31 PSI to 44PSI MAX.
People run them at the lower end for comfort. But remember this low will cause the tyre to work harder to turn as the circle is NOT a circle, but in fact a squashed circle, which is always squashed, so you use more fuel to push all four tyres that have a slight flat bit at the road contact part.


Also, different tyres and wheel sizes determines the PSI you need.
One Jaguar saying 32 PSI is no use. We need to know your tyre size and profile.
I.E. 225/50/19 on a 19 inch wheel. Lower profiles take a higher PSI most times.
this is NOT an easy thread as no correct answers, only what makes you feel safe and happy and what the tyre tells you.


Softer tyres give you more comfort, but also cause a wallowing boat effect.
Potholes seem as though they are no problem with little interference on the car which you feel through the steering wheel. The tyre acts like a shock absorber, which is actually wrong for a car driver to do.
If this is happening, then change the shocks and bushes as you should not feel anything different at higher pressures.


Higher pressure gives less road resistance/drag and greater full economy.


Look online and you will see different types of tyres that have had different pressures applied.
Tyres are designed to give under load, but too many people run FAR TOO LOW around the world on tyre pressure.
This is safer for most driving conditions and better than if you over inflate them but dangerous at moderate to extreme danger at high speed as the tyres will get extremely hot, transferring that heat to your bearings, hubs, brakes etc. The tyre is them taking up the slack before you get a chance to turn etc so it will feel like your steering is having problems.


Follow what the tyre says and not what people tell you as the tyre company has spent millions developing the tyre and technology that goes into them.


I run Pirelli P6000's on 19" rims, low profile.


No car tyre (Off-road is different), should ever have a weight bulge at the bottom of the tyre that contacts the road. This is under-inflated and dangerous at medium to high speed and causes a lot of cars to suddenly become uncontrollable in an emergency.


Put them as a rule just below the MAX tyre pressure the tyre says and not what others say it should be.
Then release air all around if you find it too hard and bumpy, say about 5 PSI on all 4. (Remember the spare may be a different pressure to your other 4 tyres, if a space saver wheel).


If you drive solo a lot then you are not putting so much weight on the tyre so less PSI will not be a problem.
But then you fill the car with shopping and people, you are expected to have more air than a solo rider to compensate for the extra weight/load put on the tyres.


Too many older people are stuck in the past and never came forward with the technology that tyres have gone through and will still tell you 32PSI as a standard which went out 30 years ago.




Even if the car manual recommends something, do not follow this as law as the tyre manufacturer KNOWS more than the car maker.
Tyre companies will work with car designers for the best tyre and rubber for particular cars but not on mass production cars.


Car makers will recommend a tyre only because of back-hand deals or deals from particular tyre makers BUT this is NOT real and the tyre company will tell you the best tyre for your car and driving condition better than the car maker/seller/manual.


I hope this helps and others add to it.

 

I really do feel that 'Oldish git' is giving some seriously misleading advice, and I find myself in serious disagreement with some of the things he has said.

1. The maximum pressure given on the side of the tyre is nothing to do with the correct tyre pressure when it's in service; it's not a maximum operating or service pressure. 'Maximum pressure' is simply that, the maximum ... because if you exceed that pressure you risk damaging the tyre internally. So if it's the spare and you keep it above the service pressure (as many of us do) don't exceed the maximum pressure on tyre; that's all it means.

2. "One Jaguar saying 32 PSI is no use. We need to know your tyre size and profile."

If he means a single figure is no use (I think that's what he means) then may I simply point out that the 'sticky' inside the fuel filler flap (on mine at any rate) gives pressures for 18"(petrol), 18"(diesel), 19" and 20" wheels, [Edit correction] for speeds below 100mph and speeds above 100 mph. [end edit] That's eight options.

3. As to

"No car tyre (Off-road is different), should ever have a weight bulge at the bottom of the tyre that contacts the road. This is under-inflated and dangerous at medium to high speed and causes a lot of cars to suddenly become uncontrollable in an emergency."

that seems to me to be utter rubbish.

That may have been true in the old days of cross-ply tyres but for radial tyres there's nothing wrong with a slight bulge, it's perfectly normal.



Car manufacturers, and tyre manufacturers, go to great lengths to give the best advice on tyre pressures. This is always some variation permisable if the tyre shows signs of over-inflation (by wearing faster in the centre of the tread) or under-inflation (wearing faster on the shoulders of the tread). However, over-inflating by 5 to 10 psi above the recommended pressure, or to the point that there's 'no bulge', will mean a considerable reduction in the area of tread in contact with the road, with a consequent reduction in grip.

Certainly in the UK, if the 'boys in blue' (sorry, the police) were to find your car with tyres with cold pressures 5 to 10 psi above the manufacturer's recommendations you could be looking at an endorsable (points on your license) offence.

The best pressures, from a safety and legality point of view, are those recommended by the car manufacturer unless the tyre manufacturer specifically recommends a different pressure for that marque and size of tyre on the vehicle in question. (I seem to recall that back in the '70s Michelin recommended different pressures for the Michelin X tyre, on the Mini certainly).


Dave In Gloucs

 

Excellent points.

Beware those who believe they 'know better' than the OEM without any real credentials to back up their claims.

 

Tire pressures are specified in Jaguar's documentation and can be reviewed here;

 

I follow what Jaguar recommends (0-100 mph) of 28 in the front and 30 in the rear for my 235-50-18's. The tires, Michelin's, have a max pressure of 51 psi, which does seem very high to me. The tires are designed to fit hundreds of different vehicles. I think the car manufacturer has a better idea of what the pressure should be for that individual vehicle. But a poor wear pattern would cause me to adjust the pressure a bit.

 

Just realize the difference between the US/Canadian pressures and the ROW is primarily due to overall road conditions.

 

I used to keep it within Jaguar's recommended pressure for 19" wheels, but suffered great mag wheel and tire damage due to potholes.

I then decided to keep all wheels the same, about 33psi in winter months due to hardness caused by outside cold air, and 36psi in summer due to heat softening the tire in summer.

Though the ride was firmer, fuel improved but ride was a little tough and needing some getting used to.

I don't know if this is related to tire pressures, I fixed my suspension 2 times within 140,000miles. The first time I took it to Jag and they replaced the front control arms and air suspension. The second time, I took it to a private mechanic who only replaced all the front bushes, including anti rollbar's.

I'm caught between a rock and a hardplace becuase our roads are literally very bad and what I save on wheel alignment, balancing, tires and rim damage, I pay in the longer run in suspension bushes, unless you believe their wear and tear is normal.

 

Another thing to keep in mind. Rumaging thru my tools/supplies I found 4 pressure gauges, two of which are old and fairly in-expensive but identicle dial type gauges. All 4 give completely different readings, some as much as 4 lbs different. I also have what appears to be a pretty decent quality dial gauge (came in a road-side repair kit x-mas present) and it's reading a bit lower then all the others. 5 gauges and 5 different readings. Gas station gauges are also notoriously in-accurate so I've decided to go with the gauge on my plug-in air compressor. At this point I have no idea what the actual pressures are. But I'm not losing any sleep over it.

 

It must be the tightness of our the Weights & Measures regs (actually part of the local authorities, and now called 'Trading Standards') but fuel pumps and air lines in the UK get inspected for accuracy every so often - there's usually a little sticker to say when it was last done - and I've always found ours fairly accurate. In the 'Land of the Free' I guess it may be different ?

 

I always keep my pressures at the Jaguar "Comfort" settings as quoted on the tyre pressure table stuck on the inside of the fuel filler cap. It only applies up to 100 mph, but for a fully loaded car, so it's OK for my holiday trips.


I never go over 100 anyway, it costs too much in fuel, and anyway, it's illegal in Europe except on German autobahns. I did all my speed stuff over 40 years ago on motorcycles, and now I just major on comfort.

 

The fuel pumps are checked by "some" government agency every so often but not the air pressure gauges. And in the "Land of the Free" the air/water pump isn't. It costs a buck and a half*, assuming it works at all. In any event, most of them here are as old as I am. And about as usefull .

* They're supposed to turn it on for free for paying customers but the only ones that work at all are at the stations with the most expensive gas so....

 

A buck-n-half ! That's well over a quid ! Yikes ! At the local supermarket (a national chain called TESCO) it's 20p and I can just get round all four wheels if I take all the valve caps off before I put the coin in, and then put the same pressure in all the tyres; but no chance of doing the spare as well !

Some places will give you an air token if you buy fuel (not the local supermarket). I only know of one place with free air but it's sixteen miles away.

 

Much of Michigan's roads are now 3rd world/pre-industrial. Long ago I used 35 psi in my cars. Now the Jag is 31 all-around and year-around with periodic adjustment for ambient temps.

 

Thanks for all your tips. I put 32/34 in mine and the ride was a bit too firm so I'm going to try 28/30.
BTW, I bought this $50 Kobalt portable air inflator from Lowe's it works really great!

 

I found mine was very firm when I got it, but found that the dealer had put 38psi all round; to prevent flat-spotting of the tyres when standing unused for a long time, I suspect.

I reduced them to 30 all round (the diesel is a heavier lump of an engine so it's 30/30 rather than 28/30) and guess what, it was much nicer.

On a long motorway trip (like Scotland) I might put them up to 32/32, but otherwise keep 'em at 30/30.

 

I keep mine at 36 psi front and rear. It seems to make the steering and handling sharper. The ride is a bit taught but I prefer it that way.

 

Great to hear what all the above tire experts say.
One important factor not said, the pressure is measured by cold temperture reading. Read the pressure prior to driving in the morning, unless it's below 50 degrees.
Reading when hot (after driving some miles) adds 3 to 4 PSI to the cold pressure.