Tire pressure with heavy loads

Started by MaxPower, May 20, 2005, 09:31:13 AM

MaxPower

What is the general rule of thumb about tire air pressure and driving with a fully loaded car?  Is it that a few pounds should be taken out?

I'm driving 700 miles next Monday with a few hundred extra pounds in the car (moving back from college) and I want to get the best gas milage possible, which includes proper tire pressure.

I haven't checked it out but I think that Subaru recommends 2 lbs less air pressure with more people in the car.

Any thoughts?  

FlatBlackCaddy

I'm not positive but i'd imagine a fully inflated(max pressure) tire would yield the best fuel mileage with or without a load.

More weight over tires with a lower pressure would result in more friction, which in turn should lower fuel mileage.

giant_mtb

A few pounds shouldn't be taken out...they should be put in.  With a load the tires are "heavier" because they cause more friction because they are more in contact with the road...so putting in some extra pressure and making sure they don't look flat when you have the load on should give you better mileage...

JWC

QuoteI'm not positive but i'd imagine a fully inflated(max pressure) tire would yield the best fuel mileage with or without a load.

More weight over tires with a lower pressure would result in more friction, which in turn should lower fuel mileage.
You are right about more friction, but this also means higher temperature. This was one reason for the fiasco on Explorer tires. Folks were loading all their vacation gear in the back of the vehicle, but not adjusting tire pressures up to accomodate the extra load. Overloaded tires and a tire pressure already four PSI too low led to high heat and then tread separation.

Since my driving is mostly city/parking lot, I add three psi to my tires. Just enough to decrease excessive wear on the sides, but not enough to increase wear in the center of the tires. Before a road trip, I move the tire pressure back to recommended ratings. On my trips to Alabama this past year, I've been carrying an air tank. On the ride down to Alabama, I am running empty. But, on the way back, loaded with items from my Dad's estate, I adjust the tire pressures to accomodate the load.

Secret Chimp

You're going to want to add about 2-3 psi to the rear wheels. The fronts don't matter, because the passengers and cargo aren't going to add very much weight over the front wheels. For my 2002 Outback, the owner's manual says to have the rear tires at about 35 psi (this is from memory, so check your own) for towing. Make sure you aren't exceeding the maximum psi printed on the tire sidewalls.
Remember to have the heaviest objects the farthest forward; it'll balance the weight distribution and also save you from some extremely hairy and likely uncontrollable oversteer if you have to swerve hard to avoid an accident on the highway (you don't want too much weight behind the rear axle line)


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Tom

Guys, does 2 or 3 psi here or there really make a difference?

JWC

I don't think two is much of a difference. Three seems to be the benchmark for, at least for me, on street cars.

But, yes, 3psi can make a difference...especially in city driving like I do. It does decrease the wear on the outside edge of the tires. I was pretty skeptical myself, until it  was proven to me by an old front end mechanic.  

I see the same customers over and over.  Some want the extra 3psi...others just want the what the factory spec is.  I've seen how just 3 psi can extend the life of a set of tires.

Under loads, that extra psi keeps the tire's temp down. Very important.

Tom

Interesting.  I think the Buick recommends 33psi, so with fuel economy in mind I usually bring it up to 40.

BartsSVO

I always inflate my tires to the pressure specified on the sidewall of the tire and take a little out if I notice any adverse effects. Most auto manufacturers specify tire pressures that give priority to ride comfort over fuel economy and load capacity. So they'll reccomend a pressure that is usually the bare minimum for the stated load rating of the vehicle just so someone doesn't bring a car in complaining that it rides too rough.

I'm sure there is some safety margin in the pressure listed on the sidewall, but I wouldn't exceed it by more than 2-3 psi. Also remember that is for a cold tire! Usually tire pressure will rise by as much as 5-6 psi when the tire has reached its ideal operating temperature
--Bart

1986 Mustang SVO
1995 Ranger XLT

JWC

QuoteI always inflate my tires to the pressure specified on the sidewall of the tire and take a little out if I notice any adverse effects. Most auto manufacturers specify tire pressures that give priority to ride comfort over fuel economy and load capacity. So they'll reccomend a pressure that is usually the bare minimum for the stated load rating of the vehicle just so someone doesn't bring a car in complaining that it rides too rough.

I'm sure there is some safety margin in the pressure listed on the sidewall, but I wouldn't exceed it by more than 2-3 psi. Also remember that is for a cold tire! Usually tire pressure will rise by as much as 5-6 psi when the tire has reached its ideal operating temperature
That's a brave thing to do considering the number of tire sidewall failures I've seen recently. Even with Michelin.  

bobwill

Look up the load rating for the tires you have.  If the total loaded up weight is getting close to the maximum weight those tires are rated for, then add a few psi.

BartsSVO

QuoteThat's a brave thing to do considering the number of tire sidewall failures I've seen recently. Even with Michelin.
What is? Running them at the pressure on the tire? Or running them over pressure? I would agree running them overpressure is but I've never experienced any type of sidewall failure in 11 years from filling the tires to the pressure on the tire.
--Bart

1986 Mustang SVO
1995 Ranger XLT

MaxPower

in the end i just went with the recommended psi.  i got decent gas mileage too, considering the amount of extra stuff i had packed away.

JWC

Keep in mind a number of factors are calculated to give you the best ride, the best fuel economy, the best handling and the best tire wear, overall,  when psi is calculated by the factory.

To give you an example, we called a factory rep into the GM dealership I worked at before,  for a Yukon that was all over the road when braking. We had checked every cause we could think of...save one. Everytime you braked the vehicle fish tailed practically out of control.  

When the rep arrived he walked around the truck while reading the ROs of what we had done to try and correct the problem. He opened his briefcase, pulled out a tire pressure gauge and checked tire pressures. He let out air, dropping the psi. Then drove the truck. He came back and said it didn't fish tail when he braked. I road with him, the service manage drove the vehicle with him and the tech who had worked on it drove it and verified the concern was no longer present.

I spoke to the rep before he left and asked how he determined the tire pressure was too high....the one thing we hadn't considered. He chuckled and said he "cheated". He had just run into the same thing on a Camaro the month before in South Carolina. It took him all day to figure it out then. His thoughts...people see "Max Press 45lbs" on the sidewall and inflate their tires to that, not the factory recommendation. A vehicle's weight is taken into consideration when tire pressures are calculated by the manufacturer. Overinflation causes adverse handling because the tire doesn't have the amount of "give" intended.  Bounce a basketball sometime with differing amounts of air and you'll see what he means.

If the tire is over inflated and can't "give", the suspension has to work much harder, causing excessive wear and tear on steering and suspension components.  Correct tire inflation not only makes for a more comfortable ride for the occupants, but for the car as well.  

What can't absorb the impact has to break. Hit a pot hole with an overinflated tire, the sidewall will give and burst. With too much tire pressure, the sidewall can't flex and absorb the energy from the impact. That, or your suspension will break.

Maybe because I see thirty-forty cars per day, I'm more likely to see faulty tires and blown out sidewalls. Can't say, but I can say that I've seen a lot of damaged vehicles  from overinflated tires and have cured many handling complaints by simply adjusting tire pressures down.

Under inflation is just as bad, something most people don't  deny.

I had a customer in last week with a conversion van. She wanted an oil change....and by the way, I want my tire pressures to 41 psi.  No problem, I knew that her van's recommended tire pressure was higher than normal. The oil change guy (I refuse to call them "lube techs") walked to my desk and said he didn't think we should adjust the pressure to 41lbs.  Reason? The sidewall max was 35psi.  Seems she had tires installed at Sears and they sold her tires not rated for her truck. But, they were Michelin LTs.  She called this week and related that they told her to run them at 35psi, but at 35psi, her van "wallows" in turns...probably due to the sidewall rolling over under the weight. If you have ever driven a conversion van they handle like boats anyway, so anything more feels dangerous and probably is.  She protested after trying it over the weekend and the manager finally relented and sold her the correct tires.  Pro-rated for the miles she had put on them of course.

cozmik

I have found with my car, addind an extra 4-6 lbs of pressure over factory helps with with the handling and the wear. It even helped with the ride with my Continentals. Max was not good for my car though. I remember when I first got my Conti's installed, the car felt like it was on its tippy toes, turns out the tire inflation was at the sidewall max. I took out about 5 lbs and all that oddness was gone.  


2006 BMW 330xi. 6 Speed, Sport Package. Gone are the RFTs! Toyo Proxes 4 in their place

BartsSVO

Now I'm a tad confused JWC. On the sidewall, it clearly states a given load rating when inflated to the pressure indicated on the sidewall. It seems to me that the weight of the vehicle wouldn't have anything to do with that number as it is a measure of how much load the tire can support irregardless of what kind of vehicle it is mounted on. I do understand that overinflating the tire over the sticker on the car can cause handling and tire wear issues on some vehicles, but I generally keep an eye on my tires for abnormal wear and I've never had an issue with it.

I still have the general opinion that most reccomended tire inflation specs are more for a comfortable ride and not because it would upset the suspension. This was actually the case in the whole Ford/Firestone fiasco where Ford was suggesting a less than safe tire pressure to keep an SUV built on a truck chassis from riding like a truck.  
--Bart

1986 Mustang SVO
1995 Ranger XLT