# Should I over- or under-inflate my tires when towing?

We're about to take a trip across the country towing 5000lb (2300kg) with our SUV. It's rated for 9k, so I'm not worried about that, but I'd like to know how much to inflate my tires.

Advice on the Internet is all over the place:

• Some people recommend under-inflating your tires, since the PSI will increase under heavier load
• Some people recommend over-inflating your tires, since the load will flatten the tires a bit
• Some people say you should use the recommended tire pressure, but they're split on whether you should fill up the tires with or without the load.

What's the actual best practice, and why?

• What the vehicle's owners manual provide any guidance? – David Jun 29 '18 at 19:21
• The label on the driver's door jamb will show normal pressure recommendations and maybe towing inflation too. if it doesn't show tow inflation then inflate to pressure indicated on the Vehicle Certification Label on the "A" pillar. – Jimmy Fix-it Jun 30 '18 at 20:49

The conflicting advice is confusing to sort out, because the logic seems to have got the cause-and-effect connections wrong. So let's start again...

The tires are the only part of the vehicle that is in contact with the road, and that contact has to support the weight of the vehicle.

The maths behind that are simple: weight supported by a wheel = (contact area of tire) x (tire pressure).

The contact area is critical for the vehicle handling. Too small, and it will tend to understeer, it will be easier to spin the wheels, and it will take less braking force to lock them. Too big, and the tire sidewalls will have to flex more as the tire rotates, which will overheat the tire.

So, the answer to your question is to find how much down force gets applied to the vehicle through the towing hitch, and then adjust the tire pressures to match that change and keep the contact area the same. Note that the downforce will affect the rear wheels more than the front, though it may actually reduce the download on the front wheels by trying to tip the vehicle's nose in the air - in which case you might need to reduce the front tire pressure. (But that is only likely to be the case if you are trying to exceed the vehicle's rated maximum towing capacity.)

Note, the down force change is not the same as the weight of the trailer being towed, assuming the hitch and drawbar are the right ones for the job. You may need to adjust the drawbar to "level" the trailer and get the recommended down load - the wrong value can make the trailer "snake" at high speeds, for example.

The vehicle handbook is the best source of the correct pressure changes, not "some guys on the internet." Getting it wrong could be life threatening!

As another answer said, you need to change the pressures when you hook up the trailer and restore them as soon as you are done. Don't drive the vehicle for long distances or at high speed with the wrong pressures when not towing!

Nothing of "proof", but you need to keep them at normal operating pressure before you load the vehicle.

You don't want them under-inflated, as while when you put a load on them and the pressure will increase doing so, this doesn't do anything to support the sidewalls. This has to do with volume, not pressure. When the sidewalls are squished (lower than normal), this causes heat to build in the sidewalls, which can cause tire failure.

Also with lower tire pressure, the tires have a lot more flex as the sidewalls are not firm enough to do their work. This means your vehicle will suffer stability issues and could be unstable enough to cause an accident. Passenger (P - prefix in the tire size) tires will suffer from this more than Light Truck (LT) tires, as LT's have stiffer sidewalls and are designed for load stability.

As @DanMantyla stated in his very good answer, over pressurizing the tires is not a good way to go either. Too much pressure can cause a blowout. The tires are designed to "pressure up" (if you will) when a load is exhibited upon them. With the proper tire pressure before loading, if you do not go past the tires load rating, you shouldn't have to worry about the pressure of the tire ... it is designed to take it. It's when you overload them you stand a chance of tire failure.

I would suggest if you checked the tires cold, before and after loading, you probably would only find a couple of PSI difference between the two, if that.

Any which way, my suggestion for the reasons above (and as Dan has suggested), it's best to just ensure the tire pressures are correct before loading and you should be golden.

I think the terms you're using - over-inflate and under-inflate - aren't the best descriptors.

There's a maximum tire pressure, around 50 psi for many tires. Inflating past that PSI would be over-inflating - but I don't think that's what you mean by over-inflating. I think you mean, inflating them past what most people inflate them to - around 30 or 35 PSI. (If I am incorrect about this assumption, say so)

Why do most people inflate to less than 35 PSI when the max tire pressure is higher? Because higher tire pressures provide a harsher, bumpier ride. Lower tire pressures give a softer ride, but at a reduced fuel economy.

You should never inflate past the maximum tire pressure.

If it were me, I'd inflate the tires a little past what you're used to - say 40 PSI. Of course check your max tire pressure and don't get too close to it. Why would I do that? To reduce the rolling resistance. This will help the engine and transmission pull the weight a little easier, which might be important if you're driving through steep, hot terrain like Nevada or something.

I don't see any reason why you would want to inflate them below normal. Unless you're worried about braking power, or you're crawling through very bumpy terrain - like washboard dirt roads. Even then I wouldn't risk it, with the added weight, you'll loose stability.

That said, I don't have a whole lot of experience towing heaving loads, nothing more than 2,000 pounds, but I know that I would rather have more pressure in the tires than not enough!

• You write "because higher tire pressure provide a harsher, bumpier ride". As far as I can tell, most vehicles have a recommended tire pressure, usually stated in the driver's door placard. – Martin Argerami Jun 30 '18 at 0:55