Re: Tire pressure vs ride

The sticker the car manufacturer puts on the door regarding tire pressure is only for the tires that originally came on the car. When those tires are replaced, you should go with the tire manufacturer's recommendation on tire pressure. The weight-bias tire pressures between front and back might be different also. You can ball-park the pressure based on the max pressure on the sidewall or you can take the car to a grain elevator/truck scale and weigh the front and back axles. Then look up the recommended tire pressure for that weight. The tire dealer should have the pressure chart.

Jack

I can't find any. Tire dealer had no recommendation other than "30 pounds each", which they recommend for all cars to all people (great, huh?). If you can find a recommendation for TripleTreds on an '89 Grand Marquis I'm all ears.

The weight on my rear axle is definitely less than the weight on my front, I'm pretty sure even with the trunk and passenger compartment loaded to capacity. The manufacturer specified a bias of 4 PSI rear for all loads.

I know of one vehicle for sure where the factory recommended pressures aren't the best to choose even for the OE tires and that car is the '01-'03 Prius. The factory recommends 35 front 33 rear but many owners have noticed underinflation wear patterns such as edges wearing faster than the center at the factory pressures and have used higher pressures instead such as 40/38 or 42/40. Some have even foolishly exceeded the sidewall max cold ratings but I have not yet heard of anyone showing overinflation patterns such as center wearing faster. A side bonus to this seems to be increased fuel mileage

__________________ Not if you owned a 1996 Ford Contour GL as I once did, size P205-60R15. Front: 31psi Rear: 34.

Try ta figger that one out!

-CC

Understood. But just because one plus-sizes their tires, or, keeps the same size tire but goes up a speed rating or two - IE from S to T or V-rated, does not mean all common-sense goes out the window and the new tires are automatically set to the max. pressure placard on the tire. I just had an alignment done and the car was riding smooth - too smooth, in fact the tires were barely gripping the road! (sure enough, the alignment shop had inflated them to 35psi - I checked - they were 38-39psi hot). So basically NOBODY in this country knows how to set tire pressure!

I know my "new" Malibu(Saab-based) has a pretty hard suspension and actually rode better with the original S-rated tires. But I went ahead and got H-rated tires of the same spec. With the S-rated tires, I could inflate them up to 3psi over the door sticker(30psi), improve my rolling, and still have a smooth supple ride; with the H-rated Yokohamas - 31psi. Anything higher and the whole damn car rattles like Arlen Specter's jaw!

-CC

Do you know who manufactured the tires? If so, contact the manufacturer and ask for a chart.

I keep 40 front and 35 rear on my 2000 Grand Marquis. That seems to give the best tire wear but the ride is a bit rougher. The tire sidewall list 45 psi at max load.

Jack

Back in the days when dinosaurs which came from Detroit ruled the Earth, it was pretty much a given that the recommended pressure was way low for reasons of ride comfort, and "serious" drivers would pump another five or so pounds in. The advent of compacts and Japanese cars has made this wisdom unreliable, but by the same token, you still can't rely on the recommended pressure to be the best performance (excluding comfort) wise. A little trial and error may be in order.

Elsewhere, I saw a rule of thumb **for an approximate starting place** for trial and error: Pressure in PSI= Total weight of car/100 + 2 lbs extra on the heavier end of the car +

(Notice that trial **and error** should take care that the error part isn't too bad.)

That said, my experience is that with pressures at recommended or a few lbs higher, I've never seen excess wear on the centers, indicative of overinflation.

Interesting; normal recommendation is more inflation to prevent hydroplaning; might be a difference in tread pattern. I assume your tread pattern is pretty "aggressive" and therefore probably doesn't need help channeling the water out, compared to the close-treaded allseason car tires?

Might be a handling thing; more pressure in the rear = more grip in the rear = more understeer than oversteer = safety on the road; and, no offense, but I would guess the Grand Marquis doesn't get inherently excellent handling from chassis design, so tire pressure tuning is probably in order.

On Oct 26, 2:20 pm, Ray wrote:

Indeed; the "story" on the FordFirestoneFiasco was that lower than optimal pressure was specified specifically to reduce tire grip, in order to prevent getting enough side Gs to tip the beast over. Unfortunately, when that came up against a run of tires on the low side of the safety margin for abuse, due to management problems at the factory, they blew.

"For example, the documents show that Ford lowered its recommended tire pressure on the 15-inch Firestone P235 ATX tires to improve the Explorer's stability after engineering simulations showed it failing J-turn emergency-avoidance maneuvers in 1989. The lower tire pressure was considered a critical component of the Explorer's safe handling, enough so that Ford weighed adding a warning sticker inside the vehicle alerting occupants that a tire inflation level of 26 pounds per square inch was "required" to help prevent "loss of control, rollover and serious injury." The warning labels were never added, however. A Ford spokesman said Tuesday that the company made design and suspension modifications in the final months before the Explorer's February 1990 launch that obviated the need. Ford considered and rejected several major design changes, including widening the Explorer by 2 inches to lower its center of gravity. Instead, Ford lowered the vehicle by one-half inch and stiffened its front suspension springs. "The engineering team felt they had achieved their goal of becoming a safety leader, so there was not the need for that warning label," said Ford spokesman Jon Harmon. Nevertheless, Ford's decision to lower the recommended tire pressure on its Explorer tires to 26 pounds per square inch, from 30 pounds per square inch recommended by Firestone, is expected to figure prominently when a fourth congressional hearing over the Firestone tire crisis begins tomorrow in Washington.

-Role of Ford Explorer Design Is Studied In Connection With Firestone Tire Suits THE WALL STREET JOURNAL September 20, 2000

I can see too low also allowing water to get under the center of the tread and causing trouble.

I do have large tread lugs which channel water although some folks cuss these tires in the wet. They behave for me. Most seem to be running them too hard though. They seem think larger tire means larger pressure.

We use a 'chalk' test to see. That is running some chalk marks across the tread and drive a short bit straight, then seeing how much chalk is left. On my style of tire, the best footprint for traction and handling leaves about 1/4" on each edge not touching.

Running either too low or too hard is pretty bad on large tires in particular and in certain conditions can cause big trouble.

I even remember the Volvo 240 GLT I used to own also didn't like hard tires either, they would hydroplane bad. They were 60 series, so 'wide' tires.

Mike

Oh yeah, good idea; I'd forgotten about that. A while back I got enamored of trying to take the tire temps across the tread like the pros do and wasted a lot of time and effort trying to get usable readings with the crappy equipment I can afford on no budget; should have remembered the simple old techniques.

_____________________

Just because one plus-sizes their tires or goes up a speed rating(IE from S to H or V rated) doesn't mean to throw common sense out the window and fill the tires to their maximum pressure as listed on the tire.

My 2005 Malibu(Saab based) came with S rated tires. The door placard said 30psi. I could go as high as 33 and still be comfortable with them and get better gas mileage. I recently upgraded to the same size tire, a Yokohama, but H-rated. Now I can go up only a pound or two over the door placard pressure. The Malibu has a hard suspension already, and going significantly above GM's recommendations is like riding the Coney Island Cyclone. Fun, but not as fun. 31psi is where they'll stay.

-CC