Air Pressure

One of the questions we get asked the most is "What air pressure should I use in my tires?" It is a simple question, but the answer hinges on what you drive, where you drive, and how you drive it. So here are some of the principles we used to determine the answer.

Preface:

California passed a law in September 2010 concerning air pressure. It roughly states that: in order to reduce greenhouse emissions caused by underinflated tires, all auto shops are required to set the air pressure in any car they work on to the vehicle manufacturer's specifications (usually found on the door placard). That's all well and good, we want to decrease greenhouse emissions, but there's a problem, for easily more than half the vehicles we work on, the vehicle manufacturers specifications are either patently too low (for this area), or barely adequate. So in order to reduce greenhouse emissions caused by underinflated tires, we were required to underinflate them(!?). Fortunately, they soon realized this, and issued a clarification to the law that they would allow us to put more air in the tires if the customer requested it. We are still not allowed to go lower.

Examples of air pressure specifications, as found on the door placard and in the owner's manual.

General Rules For Auburn Area Driving:

Because of the curvy, hilly roads around here, we generally recommend a minimum of 35 psi for anything but 3/4 ton and 1 ton trucks. We say minimum because if you drive a lot of twisty roads, your tires will naturally wear more on the edges. So short of moving, (or slowing down) the best solutions to this are more air pressure and consistent rotations. The other thing that would make us reccommend more than 35 would be either heavy, or heavily loaded vehicles. Minivans are one instance of this, because they are heavy, and often have relatively small tires, they work the tires extra hard. Most of these can benefit from more pressure. Half ton or midsize pickups also fall into this category, especially if they are loaded heavily, you just need more air to carry the extra weight. The last instance would be the car that is driven on lots of twisty roads. The turns also tend to work the sidewalls more and wear more on the edges of the tire. In most of these cases, 40 psi should be more than enough, and since most modern tire designs will allow up to 44 psi (in some cases more) this is not a problem. If you have a heavily loaded vehicle, up to 44 or even higher is ok if the tire allows it, but this is a little high for most vehicles. An exception to this would be with extra wide, low profile tires. They tend to naturally wear more in the middle, so caution should be used when going over 35.

3/4 ton and 1 ton trucks and vans:

Unlike cars, minivans, and lighter trucks that often specify low air pressure, the heavier trucks usually specify very high air pressure. A common air pressure for one these would be 55 front, 80 rear, or 75 front and rear, or like the example above, 50 front and 65 rear. Since weight capacity is a simple function of air pressure times air volume, the higher pressure is necessary to enable these vechicles to do their job. Since we are required by law to put this air pressure in, that is what we will do, (unless of course you want more). It is also important to note that even though you want to adjust the air pressure to your own liking, you should never put less air in the rear of a pickup.

One exception to this which is very common, is for oversize tires. Since, as stated before, weight capacity is a function of air pressure times air volume, you will often see larger truck tires primarily available in lower load ratings. A common example woud be an LT265/70R17 load range E and a LT285/70R17 in a load range D (there are E rated 285s but D they are the norm). The maximum inflation of the 265 in the E range is 80 psi and the maximum inflation of the 285 in the D is 65. However, due to the larger air volume of the 285, at maximum inflation, these two tires have the exact same weight capacity (3005), down to the pound! In this case, the tire pressure can be adjusted according to charts we have.

Different Air Pressure Front to Rear?

Cars and trucks alike often specify differing air pressure front to rear. Why? There are two main reasons for this, load and handling. The specific reason could be either one or both depending on the vehicle.

Load: Whether you have a commuter car or a 1 ton truck, it's primary function is to carry people and items. Many vehicles specify more air in the back to allow for more weight to be carried. However, even if you never carry anything more than you, don't be too quick to lower the rear air pressure because of the second reason:

Handling: If you watch any type of vehicle racing, you will know that they use air pressure to fine tune the handling of the vehicle, and it is no different for street vehicles. Often, the vehicle manufacturers specify more air pressure in the back for this very reason. (Sometimes, not often, they specify a couple of pounds lower in the rear, but rarely more than 2 psi.) Some prime, but very different, examples of this would be sports cars and pickup trucks (or larger SUVs) We have seen more than one occasion (car and truck) when putting more air pressure in the rear has completely stabilized a previous "squirrelly" vehicle. Putting less in the rear may seem like a good idea on a pickup that is seldom loaded, but this can have the opposite effect of making the rear unstable, so we never recommend less air pressure in the rear of a truck. A good rule of thumb would be to keep the same variance front to rear that the vehicle manufacturer recommends. An example of this would be the Toyota Tacoma 4WD. For years, Toyota specifed 26 front and 29 rear, which is really low for this area. So if you wanted more, a good option would be 35 front and 38 in the rear. This would insure better tire life, (and better gas mileage) while keeping the vehicle handling the way Toyota designed it. On sports cars, or even luxury sedans, this effect may even be more pronounced, so you should keep the variance the same as the manufactuer recommends. If you ask: Well, won't the rear tires wear in the middle? The answer is yes, but the solution for this is not fiddling with the air pressure and potentially making your vehicle unsafe, but rather consistent rotations, usually every 5,000 miles.

Different Air Pressure Summer Versus Winter?

People sometimes ask us if they should put less air pressure in the summer. They reason that since the natural air pressure increases due to heat while driving is more pronounced when it's hotter, then it would be better to put less air pressure in them. Though this makes sense, it is not correct. Tires are designed with this in mind, and if you put less air in, the tire will flex more and will actually build up more heat! Since heat is one of the primary enemies of tires, less air in the summer is definitely not better. As far as winter running goes, we are asked if they should put more air in since the pressure drop when it's cold. The answer is no again. However, anytime your tires get low, you should add air, and since tires lose air pressure naturally over time. you should always check your tire pressure every month or two.

Nitrogen in Tires?

You may have seen retailers advertising Nitrogen in tires. The idea is that since Nitrogen is a larger molecule, it seeps out of tires slower, so that the tires will stay inflated properly longer. In addition we've heard that tires won't vary in air pressure as much when the tires get hot. Though we would not want to "naysay" the use of nitrogen, we have not chosen to incur the expense of a nitrogen separator for the following reasons: 1. 78% of air is nitrogen already and 2. Regular air pressure checks, which we recommend, negate the problems of air pressure loss due to "seepage". Again, we are not saying that Nitrogen in tires is not a good thing, but more to say that with our limited budget, the benefits don't quite justify the added expense.

Why do car tires take 44 or even 51 psi?

It may seem odd that the majority of modern car tires take 44 or even 51 psi max pressure, while most cars specify 35 psi or even less. Since we live in the foothills, we use the extra capacity to compensate for winding road driving and weight bearing, as stated above. However, the real reason for this has nothing to do with either: When cars are driven above 100 mph, the air pressure in the tire decreases, and more air must be put in the tire to compensate. So the real reason for the higher capacity in these tires is actually for driving at speeds over 100 mph, such as on the Autobahn!

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