Tire PSI

Door plate recommendations are the automaker's choice for best balance of vehicle characteristics they prefer to promote.

You're free to change them, as long as you stay within specs. With tires, that means not to exceed the cold PSI rating. 45 on a tire rated for 50 is just fine. I've been doing that with my Prius for over a decade now.

 

The factory pumps up the tires to high pressure for their Pacific Cruise, to prevent flat-spotting or whatever on that long journey. The dealers are supposed to reduce it to the door placard level during PDI, but many 'forgot'. . Probably just shortcutting.

Many of our Gen3s were being sent out the door with 40 psi. I later heard of numerous Gen4s going out with about 50 psi. With the cars parked and chained down on the ship, and only barely moving on the loading ramps and adjacent parking lots, they are not really constrained to obeying that tire sidewall limit before delivery to the final customers.

I'd think "smoother" was a figment of your imagination, it really should be harsher.

As for more grip? This is a regular religious war, you'll have to choose your own side. Many of us like higher-than-door-placard pressures for various reasons (not including smoothness or ride comfort), while many others disagree.

 

2013 Gen 3 . Keep mine at 41, 40.

 

Yeah, as long as it is above the specified psi on the door placard and below the max rated on the sidewall, you are free to change and experiment. As I understand it, the underinflated tires are the more of problem and especially this time of year when the temperature drops in morning. I can have 4-5psi cold pressure difference in my tires between morning and afternoon.

I had higher tire pressure between 42 to 45psi in my Gen3 and experimented on my 2017 Prime. In the end, for Prius Prime, the tire inflation had very little perceivable effects on mpg, smoothness, comfort, or handling for my driving. I have been just using 38 psi all four corners to give a few psi headroom. I check the tire pressure every time I drive using an external TPMS, but only top off air to adjust back to 38psi cold pressure in morning.

 

Anything between the "recommendation" and the max. shown on the sidewall will be fine.
Choose whatever you prefer, based on some test miles.
Most people can't tell much if any difference......in ride quality or fuel mileage.

Once you decide on "your" pressure, you might want to reset the Tire Pressure Monitor to the new values.

 

Like my gen II & III Priuses, I find that my Prime tracks better and handles highway crosswinds better when I inflate the front tires to ~2 psi over the rear tires. I use 37 front/35 rear.

 

The higher the pressure the better the economy.

Recommended to keep them at or no less than 5 below max sidewall for best tread life and good fuel economy.

Higher PSI = better fuel economy, but with diminishing returns. I run 60 PSI.

Running over 60 PSI for will likely increase tread wear but will hold fine.

Running the flatter 30-35 PSI makes the ride slightly more comfortable at a cost of poorer fuel economy, increased tire wear due to more sidewall flex, and tire heat build up.

 

Very incomplete and unwise advice.
YOU can decide to do whatever you please but in effect advising others to exceed the manufacturers max. safe pressure by 15 PSI is just irresponsible.
Extra pressure reduces the contact patch and, generally, reduces the traction.
Too much and the mileage may actually fall due to tire slippage on the driven wheels.

 

I think you quoted the wrong part of my post if you were talking about my recommendation.My advice was "Recommended to keep them at or no less than 5 below max sidewall for best tread life and good fuel economy."
Care to expound on which part was incomplete or unwise?

This is a given, although in most cases the traction is sufficient. Running tires at max pressure should not cause any problems, otherwise the manufacturer would not be able to spec them as such. Higher pressure is at your own risk.

Care to expound on this? Some people run 100 PSI and do not experience slippage. Of course, it is quite risky running such a high pressure and I wouldn't recommend it. We aren't running bare metal wheels with no rolling resistance, and the Prius isn't a race car, so your suggestion of slippage is a bit absurd given the type of car this forum is about. If you're driving hard enough to spin the tires, then you need to evaluate your driving habits. Tires can spin even with <30 PSI.

 

The simple fact that you said: "I do this" implies a recommendation.
And I did not say "spin" but "slip".
But I quit.
This obviously is another case of: "Don't confuse me with facts because my mind is made up".
And / or "how dare you
disagree with something that I said."

 

It's alright, I understand that you happened to misread it, it's okay to admit that. "I do this" is definitely not always a recommendation.

Enough off topic chatter though.

 

One thing you should NOT factor into any decision about tire pressure, is what psi your particular car was delivered to you with.

NEVER inflate less than the "door jamb" recommended number.
Most car makers recommend what I find to be too low a number.
They want a "smooth ride", and they don't allow for much inflation error before load capacity safety margin is gone.
Some, like BMW, even admit it in their owner's manual, saying things like the recommended pressure is only for up to 85mph and only without back seat passengers, to add 6psi/whatever if you do those things. Ridiculous.

I buy only tires rated for at least 50psi now (typically 51psi) and personally I do inflate mine to 50psi cold at all 4 corners. I used to keep the recommended front vs rear pressure differential, but have found that at higher pressure, I personally prefer equal pressures. Sometimes a pressure differential is specified just to keep load capacity safety margin, other times it's to achieve the car maker's preferred amount of understeer vs handling quickness.

Unless it violates the "max pressure" listed on the sidewall, I'd never inflate a standard load passenger tire (SL or unspecified) to less than 36psi. Likewise for XL, 42psi minimum, they don't reach their designed load capacity at as low a pressure as SL tires.

Michelin X-Ice3 XL tires last 63K miles per set on my Prius Prime, so I can confirm that anyone claiming that pressures above those on the door jamb, would hurt tire life, have it precisely backwards. Likewise, it handles like it is on rails. At steady state, lower pressures can get maybe 5% more traction, but in transient maneuvers low pressures are unpredictable and dangerous. That's before even considering that higher pressures give you more time to notice a slow leak before load capacity safety margin is gone.

If particular tires on a car get too twitchy for a particular driver's skill level at higher pressures, figure out what is the highest pressure where every driver of the car loves how it handles. Low pressures are just burning money on extra gas and shortened tire life.

 

Many great insights in the above replies.
I would lower to the OEM suggestion (or 1-2 higher - in regards to MPG) and monitor wear.

FWIW.
Set to the PSI when the tires are cold (for example 36 psi) - first thing in the morning - and look for a 10% increase in PSI when driven and the tires are hot (40 psi in this example).
The 10% increase means you are in the ballpark for a correct pressure.

 

I never measure or care about what the pressure is when rolling down the road with the tires heated up, whether from driving or from weather. It doesn't matter, unless one tire is acting differently from the rest due to a defect like a broken belt making it smoking hot. I'd never count on a pressure increase due to tire heating, to save me. The tires have to be ready to support the load plus a safety margin, at the coldest temp and lowest pressure they will see between the last time you aired them up, and the next time you air them up.

The reason cold pressures are specified, is that inflation even a fairly modest amount (say 6psi) below spec is often downright dangerous, and inflation the same 6psi above spec is rarely if ever dangerous. They are basically admitting that however high the pressure goes due to heating (whether due to weather or tire heating due to driving) it is OK, while underinflation is not.

The load capacity safety margin at spec pressure is hopefully a good one, but I know for a fact that it is sometimes marginal, which is shocking considering how many lawyers and engineers car companies employ. If you consider that, in combination with tables of tire load capacity versus pressure, and tire pressure versus weather temperature, it becomes obvious that people can get into danger, plus get bad mpg, plus wear out their tires prematurely, quite easily while intending to keep their tires around spec, by not paying much attention to things like whether their tires were fully cooled off when they were aired up, what the weather was then versus the lowest it will be before they check/fill again.

There are great tables/charts of these things on tirepressure.org

While there are exceptions to every rule (cars with really specific inflation needs, often due to having different tires front and rear) almost every passenger car would be better off at 6 psi above spec, if also respecting the max psi stated on the sidewall. Plus using the lowest max psi of mismatched tires, on an axle, which I simply never install.

 

The tire's maybe be better off, but the suspension takes more shock? And your fillings... You make a lot of good points though.

 

I've ignored the front to rear differential in the past, and it lead to faster wear of the center tread of the rear. Check the tread depth regularly across the tire width, to just the pressure.

 

True, by reducing tire mushiness, you do transmit more bumps/vibrations/shocks to the car and eventually yourself. I think my fillings are safe, though, because everything from the seats to my butt to my spine have suspension characteristics of their own. Each has their "suspension travel" limits, but I don't recall ever bottoming out the seat cushion to the vehicle floor.

High psi in the tires can slightly increase tire puncture probability when driving over sharp rocks, nails, etc, due to the same weight being carried by a smaller tire-road contact patch. I've had only one flat where it may have contributed, and that is with about half of my million estimated miles driven, being since I went to a max-psi strategy. In the half where I used more of a standard pressure, I had one equally suspicious flat that I would have blamed on a small, high-pressure contact patch, except that I did not have that at the time. So, my own experience shows little evidence of it making a difference, though it theoretically should.