Proper Tire Inflation

tires

#1

Several years ago, when steel-belted radial tires hit the market, I heard the recommendation that an owner should fill the tires to the cold rating on the tire sidewall, rather than follow the vehicle tag recommendation, to gain better gas mileage. I had followed that for years, and it usually meant the difference between 28 to 30 psi (vehicle tag) and 35 psi (tire sidewall).



I recently examined the tires on my car for the sidewall pressure, and it was a whopping 45 psi. The vehicle tag recommends 28 psi. I asked the tire dealer, and he said that I should use the vehicle tag recommendation, since that considers vehicle weight. He said that a higher pressure could adversely affect braking and tire wear. Inflating to 40 psi enhanced my gas mileage about 5% to 10% on the highway.



What is right here? I run the wheels off my cars, driving about 35,000 a year on one car alone (we seem to average more than 55,000 a year using about 2000 gallons a year between two drivers on all three). Thus, it makes a difference when it comes to mileage and tire wear.


#2

Follow the recommendations on the vehicle tag, not the tire’s sidewall. Actually the manufacturer’s recommendation is a compromise between a number of factors such as ride comfort, tread wear, handling, safety, and so on. Since you are concerned primarily with tire longevity, inflate your tires to 2-3 psi above the tag’s recommendation. That’s what many of us do.


#3

SteveF is right on target.

I would add that the tires on your car might be appropriate for a score of car models, each with a different weight and a different weight distribution. The inflation pressure listed on a tire sidewall is the maximum safe cold inflation pressure for that tire, NOT the recommended pressure for all vehicles using that tire size. In fact, inflating your tires to the pressure listed on the sidewall could induce some dangerous handling qualities, particularly the tendency to excess oversteer. So whoever gave you the advice that you have been following all of these years was wrong, wrong, wrong.

In addition to giving you a harder ride, the high inflation pressures usually lead to excess wear in the center of the rire tread, rather than having the wear evenly distributed across the tread. Also, the higher inflation pressures make it more likely that you will cause premature wear on your struts and on front end components like tie rod ends.

As Steve said, after finding out the real recommended inflation pressure for your tires (from the Owner’s Manual or from the label on the door frame), adding about 3 lbs. to those numbers will give you slightly better handling and slightly better gas mileage without the downsides of a very harsh ride and accelerated wear on front end components.


#4

How it feels while driving on the freeway and mileage is important, but personally I am most concerned on how it will handle if I need to make a sudden turn or stop while traveling at highway speeds on wet or dry pavements which may be of different materials. I am more than willing to give up a little mileage to increase safety.

I want the tyres working together with the suspension of the car to provide a safe ride.

If you check around, you will find that different models of tyres of the exact same size will have different maximum pressures listed on the tyres. I don’t believe it is much of a stretch of the imagination to realize that they don’t mean that the same size tyres should require different pressures on the same car.

It should IMO be clear that when it says maximum on the sidewall it means that the the highest pressure that tyre can safely be used at (cold tyre pressure) There would have been no need to add the word maximum if that was recommended or ideal pressure. Also note that since that tyre is likely to fit many different cars, it is also not a stretch to realize the the ideal pressure for one car is not likely to be the same for another.

Look at the owner’s manual for the proper pressure. A few PSI over is OK, better at little high than a little low, but not more.

Note on tyre pressure:

Higher pressure will increase mileage. However if you start with the car manufacturer’s recommended value there is not much more to gain.

The car manufacturer test the tyres on your car under a number of different conditions and test for safety under emergency conditions before recommending tyre pressure.

The pressure on the side of the tyre has not been tested with your car or any car. It is only a measure of the amount of pressure that particular tyre can be safely run at, assuming that the pressure is safe for the car.

The only real use of that number on the side of the tyre is to tell you that if it is less than the car manufacturer calls for, you can't safely use those tyres on your car.



None of the above proves that it would be unsafe to use the 85-90% of the max on the tyre.  It only proves that it has not been tested.  I prefer to have my tyres (and the other guy's as well) inflated to a pressure that has been tested and found safe for handling under emergency conditions that I hope I never have. 

References:

http://www.tiresafety.com/

This is sponsored by a tyre manufacturer Bridgstone - Firestone and instructs the reader to check the owner’s manual for the correct tyre pressure.

Here is another from the Goodyear site.

Check Your Air Pressure
Keep your tires properly inflated and you could improve gas mileage by more than $1.50 every time you fill your tank. The recommended tire pressure for your vehicle is located on a sticker inside your driver-side door or noted in your owner’s manual.

You might also try the US Department of transportation.

http://www.nhtsa.gov/cars/problems/E...intenances.htm

They say:

You can find the correct tire pressure for your tow vehicle in the owner?s manual or on the tire information placard.


#5

www.carbibles.com has an excellent primer on all the basics of wheels and tires. I recommend a visit.


#6

I use 34 to 36 on all my vehicles and I get great mileage, gas and tires. I would consider the vehicle tag as the minimum pressure. Since you loose about 1 psi per month, you should add 1 psi for each month between the times you check the tire pressure, and no matter what, don’t let that be longer than three months.


#7

From personal experience, I’ve found that the pressures on the door sticker are weighted a bit too far toward comfort. Most new cars will come with 28psi because it makes the new car ride more softly which is what many buyers consider when test driving a car.

I drive performance sedans which use higher speed-rated tires than the average and which also use higher pressures. My W-rated tires have a 44psi max pressure rating and my wife’s car H-rated tires have a 38psi max pressure rating. Also note that many cars may have the generic car body sticker, but have been upgraded to a better handling package during manufacture, so I’d trust the tire before I’d trust the sticker.

I’ve found that the best combination of handling, fuel mileage and tire wear comes around 4-6 psi less than the maximum rating for the tire. That’s a good starting point but make sure you measure tire pressure with an accurate gauge and COLD in the shade. Make sure your alignment is spot-on, then watch the tire tread very carefully to see if you are getting center or outer tread wearing faster than the rest of the tire. Your tires should wear evenly all across the tread surface. Radial tires are much less likely to lose tread contact with pressure changes than the old bias-ply tires, so the argument of safety isn’t as valid as it once was unless you are grossly under or over inflated. Slightly higher pressures reduce sidewall flex during quick avoidance maneuvers and give you better control, as well as reducing rolling resistance.


#8

In your post you mention the diffences when steel belted radial hit the market. The bias ply tires used before then generally had different pressure recomendations. When radials first came out, they would have both a cold and a maximum pressure recommendation because what was recommended in the vehicle manual no longer applied to belted tires. There were general rules to go by but to a large extent people had to guess. There were a lot of belted tire blow outs in those days and to some extent it was blamed on the new tire technology but it was also felt folks were confused as to what pressure to use. Today’s cars are meant for belted tires and you should stick to the vehicle manufacturer’s recomendations. The maximum on today’s tires are there to say “Don’t use this tire on a vehicle whose’s manufacturer recommends a pressure above this maximum”. They no longer put a cold air pressure on belted tires.


#9

The greatest cause of excessive tire wear is using the wrong pressure.

I believe I read in a recent issue of Popular Mechanics that this month that the government is going to require all new cars to be equipped with tire pressure monitoring equipment, because of the effects on wrong pressure on the consumption of petroleum … both gas mileage and the extra use of these products in the manufacture of tires.

http://www.tire-information-world.com/tire-wear-problems.html has a simple graphic view of what improper tire pressure does.

The number printed on a tire’s sidewall is definitely the maximum pressure which the tire is designed to be use at, but it is not the pressure which the designer of the car determined was the best for the car.

Since there’s no way of knowing what kind of vehicle a tire is going to be put on, there’s no way the tire manufacturer can tell you what is the best pressure to use. Conclusion: Follow the car manual and if you’re really fussy, start making minor adjustments, up or down from that point to get the kind of mileage, handling and tire life you want.


#10

I placed a link in the above comment which didn’t come through. It was