OK…
I have 6" rims and run Dunlop Signature P205/15 tires ( great, inexpensive tires, BTW…great handling and feel). The original handbook with the car talks about the original Dunlop tire options that were available when the car was new. Those pressures, depending on tire type / use, range from 23/25 to 40/40. My tires have a max. pressure of 44.
I have been running them at “in-between” pressures, trying to find a sweet spot, and have settled on 29. I do some cruising and spirited driving, always on dry roads.
Most of the previous responses are quite old…and I’m betting there is more experience with you folks now. So what are your opinions/thoughts/suggestions for tire pressures on a street 4.2L running these slightly oversize tires ?
Harold…….it depends on usage…… street and long driving …32- 34 is in my view best …. I go for 34 because although it’s a slightly harder ride …. You get less distortion under heavy braking ( ie more rubber on road = better grip) …however racing …well that’s another story that others on this forum really know about !
soooo.I have yet to find a tire manufacturer or car side door tirr pressure label that says what the ambient air temp–or tire carcass temp should be for the psi that they recommend as “cold”. Let’s just say 68 degrees as I have seen that suggested. Tire pressure gains or loses about 1psi for each 10-18 degrees F. SO…setting cold on a nice fall morning at 40F…may be a few lb different than setting cold in summer at 80F. Now consider that driving on pavement that is 110 F in the mid day sun will heat up the tire much more than on 60F pavement. Are you driving an fairly straight roads for sustained speeds for hours? , and at what speed–or are your cornering in a sporty style on a winding road. It get crazier: park with one side of the car facing sun on a hot day, the other side is shaded…the sunny side tires can be 2-3 psi greater than the shady side. Yeah, I know—.a few degrees of tire temp–but on any one morning–drive at cold settings of 32, then re set at 28, then at re set at 36 and you WILL notice the difference. For sporty driving it can make a very big difference-and at adhesion limits drivers of F1, NASCAR and the like can feel 1/4 lb differences. I DO adjust a few psi depending on the weather of the day. I am looking for the feel to be close to the same…but it isn’t of course–pavement temp and tread rubber temp varies the grip, and feeling of road surface a lot. I notice all this a lot more in the Corvette–which I push much harder: 2 psi makes a big difference.
but ok–32 psi at 68F is a good setting unless wildly hi, or low temps. In normal driving the psi increase due to heating of the tires is about 5-6 psi after 15-20 minutes–then remains about the same unless a very hot pavement or more sporty driving.
(sources-Tire Rack, TiresPlus, Goodyear and other websites
Nick
Sixty-eight degrees is probably a pretty good guess as many organizations that have set standards for standard temperature and pressure have used that temperature or one very close to it. Another potential variable is your tire gauge. How accurate is it? I’ve got three, two analog dial gauge and one digital. At 32 PSI they all read within a pound or so, but others may vary more.
I’ve got one car that recommends 32 front and 36 rear unless I plan to drive over 100 MPH for an extended period of time. Then they recommend 36 front and 40 rear.
The car I was talking about is a rear wheel drive convertible and I assume the suggested tire pressures are to reduce the tendency to oversteer and perhaps roll over.
Hi John. I’ve been racing and autocrossing E Types for the last 35 years. The E Types come from an era when it was thought that cars should understeer - often badly. Jaguar was no different. The front suspension is set up to generate understeer, with geometry that causes the outer front tire to go into significant positive camber as the car rolls into the turn. That can be corrected with various modifications to the front, including heavier sway bars and changes to the mounting points of the upper control arm. The tire pressures recommended by Jaguar in the day - more tire pressure in the rear contributed to the understeer by increasing grip in the rear. A skillful driver can somewhat compensate for this with power on a track but you don’t want to be driving like this on city streets. Reducing tire pressure reduces grip in the rear, balancing out some of the understeer in the front, and makes the car’s handling more comfortable. Try it. Incidentally it’s probably not impossible to roll an E Type but it is very difficult - God knows I tried awfully hard!
BTW, the recommended tyre size is only available here in Blighty as a street legal racing tyre, entirely unsuited to the car.
There is only one alternative tyre, different width and profile, that is available and suitable and reassuringly expensive. I might be tempted to buy 17 or 18 inch rims and fit modern tyres intended for a high performance Range Rover or Beemer SUV.
not so sure about that…if front are too high–too firm…and rear grippy…then understeer is likely…(a lot depends on the car–weight balance, suspension and other factors…: drivers get terrified with understeer…passengers scream with oversteer.
Nick
Thanks Terry. I’ve never driven a race car or autocrossed, but I suspect the most desirable set-up regarding tire pressures are different for those activities vs those most desirable for normal driving on the street. The car I was referencing is my Mercedes CLK Cabriolet.
Actually in the case of E Types reducing the pressure in the rear reduces the grip in the rear. I’m not suggesting any of you race or autopcross, what I’m suggesting is on stock E Types a lower pressure in the rear makes it a better and easier drive on the twisty’s at speed. Try it - if you don’t like it, well put the pressure back in.
In virtually all modern cars, including front wheel drive, that is true!
You had mentioned that Wayback when in the dark ages of racing it was the opposite, and it was, and it was what I had grown up with with Dad. Modern suspension and radial tires made things like that much less prevalent.
I visualise the chart of cornering power vs tire pressure as an inverted fish-hook, where the peak represents the pressure to achieve max cornering power for the load of the moment - so changing the pressure in either direction reduces cornering power, giving an opportunity to improve the balance (or screw it up).
Departing from the optimum pressure, of course, reduces overall cornering power in the quest for balance - not always the ideal solution, hence the attraction of deeper changes such as Terry mentioned above.
I’m using the term cornering power instead of grip because few of us spend much time in the sliding region, so we’re mostly interested in the stiffnesses and movements that add up to our overall direction, and our comfort with it,
In that context, understeer is A Good Thing. It has an image of being for the wimps, but a sensible level of understeer gives you a faster and more accurate response to your steering inputs, a platform of stability from which to launch heroics. Mathematically and practically accepted for years.
My theory is that Jaguar put very high levels of understeer in the front - not just with the camber, the steering is also set up to give huge roll understeer - to cover the somewhat unpredictable range of behaviour of a rear suspension which is independent in some situations, not in others. (I’m using the term “unpredictable” in a technical, not emotional, sense. Please don’t take it as a personal insult).
It’s an interesting question Nick. I suggest it’s not a critical matter because the front and rear tyres will see similar temperature changes so the overall balance will not change. Unless you’re running against a stopwatch I think that balance is more important than the specific cornering power at a given instant.
In vague support of that lazy offering, I worked in the auto industry more than forty years, tested cars and tires on four continents and only adjusted pressures for ambient temperatures in the Arctic Circle. In fact, tyre pressures were generally not seen as a very significant tuning tool, being typically set in the early days of a program and changed only rarely and minimally.
I say get out on that hot dusty road and enjoy it at whatever pressures feel good to you.
Another valuable tool I don’t think has been mentioned yet, is taking tire temps across the tread after driving the car in the manner you wish to use it. With just 12 measurements (3 per tire) you can learn a tremendous amount about pressures, alignment etc. Entire chapters of several books are dedicated to the analysis of tire temps.
I used this approach to great effect back in my karting days.
