How much do you inflate your tires...tire spec or vehicle spec?

[SirEvan]

This is a dumb question, but my mom brought it up and I didn't have a good answer for her. How much air do you guys put in your tires? Do you inflate to what the tire says (or close to it), or do you go off the vehicle sticker on the inside of your door?

I have never done anything but what the tires state, and in most cases my vehicle sticker matches the tires anyway (except if i put larger tires on). But my mom has a Suburu outback, and while her tires say something like 50 psi, the vehicle sticker on the door says to put in something like 25-30psi...almost half! I look at her tires and they look highly under-inflated (bulging out on the bottom), but she seems to think that she should be going off what the door says and not the tires.

So what would you guys recommend? Door....or tire?

[mulligan2k]

door, the tire doesn't know the weight of the car that it could be fitted too

[BajiRav]

I always go by the vehicle spec.

[threetonesun]

But my mom has a Suburu outback, and while her tires say something like 50 psi, the vehicle sticker on the door says to put in something like 25-30psi...almost half!

Door... I can't think of any tires besides temporary spares that would be at 50 psi, unless that's possibly the max inflation. :s

[Another Canuck]

Door.

The spec on the tire is the maximum inflation pressure. Always use the spec listed on the inside of your door.

[ccoltmanm]

Tire. Highest psi possible. Best mileage possible. Right?

[tsupersonic]

Tire. Highest psi possible. Best mileage possible. Right?

Nope. You always want to be using the vehicle specs, which are usually on the door.

[Another Canuck]

Tire. Highest psi possible. Best mileage possible. Right?

You realize you're talking minute fractions of an MPG, right? You'll be lucky if you save $20 a year.

The decreased rolling-resistance of an over-inflated tire is not nearly worth the decrease in handling, comfort and stopping power, and increase in road noise and risk of blowout.

[SirEvan]

Tire. Highest psi possible. Best mileage possible. Right?

Thats what my thinking is. Plus, what if you put larger tires on? I went from 17" run flats on my 335i to 19" lower profile tires. The door says around 50psi, my tires say 51 psi. But what if it was different and the new tires had a max rating of more? then you would be under inflating. I've always been told you don't want to bubble your tires out because it leads to decreased gas milage, as well as earlier failure of the tire, and have always gone with within 5% of the max PSI on the tire.

@mulligan, I could maybe understand that, but when I buy tires I buy for a vehicle, and rim size...so it would stand to reason that those tires were made for that vehicle/rim size, and therefore would know the max weight of the vehicle?

[cybertimber2008]

Tire. Highest psi possible. Best mileage possible. Right?

Over inflating increases your risk of hydroplaning.

I go with what the tire says, but I fill to ~5psi below the max when my tires are cold.

[articuno1au]

I do tyre pressures, but I buy tyres designed for my car type.

/shrug

[ShareShiz]

Tire. Highest psi possible. Best mileage possible. Right?

wrong

.. best to do what the car manufacture says, they don't do loads of testing for ****s and giggles.

[Astra.Xtreme]

Tire. Highest psi possible. Best mileage possible. Right?

That will decrease the life of the tire pretty badly. Always go by what the car is rated for.

[Another Canuck]

Thats what my thinking is. Plus, what if you put larger tires on? I went from 17" run flats on my 335i to 19" lower profile tires. The door says around 50psi, my tires say 51 psi. But what if it was different and the new tires had a max rating of more? then you would be under inflating. I've always been told you don't want to bubble your tires out because it leads to decreased gas milage, as well as earlier failure of the tire, and have always gone with within 5% of the max PSI on the tire.

@mulligan, I could maybe understand that, but when I buy tires I buy for a vehicle, and rim size...so it would stand to reason that those tires were made for that vehicle/rim size, and therefore would know the max weight of the vehicle?

I think you're reading the spec wrong. 50psi is absurdly high. It should be somewhere in the 30-40psi range.

The 335i is a relatively light vehicle. You do NOT need 50psi of pressure in your tires.

[ANON86364]

PSI is a universal measurement of the pressure the air is under regardless of the tire. You go by what's on the car.

[webeagle12]

Over inflating increases your risk of hydroplaning.

I go with what the tire says, but I fill to ~5psi below the max when my tires are cold.

usually whatever pressure is written, they mean when tire is "cold"

[threetonesun]

The max PSI number is only there so you don't blow the damn thing up in your face, you're not supposed to inflate it that high. Over inflating a) makes the ride worse b) increases braking distance and c) decreases cornering grip.

Not to mention the one "upside" you're claiming, better gas mileage, is non-existent, rolling resistance from the tires is an incredibly small factor in gas mileage, and probably not at all affected by tire pressure.

[linsook]

I think you're reading the spec wrong. 50psi is absurdly high. It should be somewhere in the 30-40psi range.

The 335i is a relatively light vehicle. You do NOT need 50psi of pressure in your tires.

I agree, 50 is very high.

[SirEvan]

I think you're reading the spec wrong. 50psi is absurdly high. It should be somewhere in the 30-40psi range.

The 335i is a relatively light vehicle. You do NOT need 50psi of pressure in your tires.

You're correct, i just went and looked, i was reading the tires. 19"x 8.5" tires show 51 Max PSI, door sticker says 36/38PSI, although thats for runflat 17's. I found a post on e90forums that lead me to this:

(Vehicle Weight in lb/100) + 2 psi at heavier end + 2 psi all around if suspension and alignment are stock.

Example: Stock 911, 3,000 lb.

(3000/100) = 30 psi

Add 2 psi all around = 32 psi

Add 2 psi to heavy end = 34 psi at rear

With modified suspension, the result is 30 psi front, 32 psi rear.

so it looks like I've been over inflating. I duno about the door, since it's rated for 17's but I'll try this math out and let some air out, I've been inflating to around 45 PSI on 51 PSI max tires.

Thanks guys!

[BajiRav]

so it looks like I've been over inflating. I duno about the door, since it's rated for 17's but I'll try this math out and let some air out, I've been inflating to around 45 PSI on 51 PSI max tires.

Thanks guys!

Must have been a rough ride? :|

[Another Canuck]

You're correct, i just went and looked, i was reading the tires. 19"x 8.5" tires show 51 Max PSI, door sticker says 36/38PSI, although thats for runflat 17's. I found a post on e90forums that lead me to this:

so it looks like I've been over inflating. I duno about the door, since it's rated for 17's but I'll try this math out and let some air out, I've been inflating to around 45 PSI on 51 PSI max tires.

Thanks guys!

That's more like it!

As Xenosion said, tire pressure is universal. The spec is relative to the weight of your vehicle, not to the size of your tires. 30psi in a 19" wheel is the same as 30psi in a 17" wheel. It will take less air to attain 30psi in a low-profile tire, but it is still 30psi and will support your vehicle just the same.

The only reason to go off-spec is if you're trying to tune the handling of your vehicle.

When you drop the PSI in your tires to spec, I guarantee you'll feel a noticieable increase in handling and comfort, and decrease in road noise. Also, depending on how long you've been driving with your tires at 45psi, you may want to inspect them and check to see if they're worn more in the middle than on the edges.

Edit: I'm not sure about RFTs. That might be an exception for running higher PSI.

[Liquidfox]

Jesus, I'm really hoping American tyres are labelled differently to UK tyres. In the UK, the PSI on the tyre is the maximum the tyre is rated for. If you fill your tyre to this you're asking for trouble! You'll have greatly reduced grip and higher risk of blow-out. You should always fill the tyre to the car manufacturers stated guidelines +/- a couple of PSI for personal preference.

[firedog79]

Take 10% off the MAX tire pressure and this would be the tire pressure you need to fill your tires with. The pressure is based on the tire manufacture, not the vehicle recommended pressure. Each manufacture rates there tire pressure for their tire. Most of the time the vehicle and the tire will be close, but if there is a significant difference go by the tire. I have always gone by the tire manufacture.

[sc302 Veteran]

In the door jamb of american cars there is the recommended air pressure to have in the tires. I inflate to that, many places are now filling the tires with nitrogen to hold the air pressure constant, if filled with nitrogen you are better off going back to the place that filled your tires for a refill if needed. Do not fill to the maximum you will have issues with over inflation causing your tires to not have proper tread grip on the road and a strip wear pattern on your tires.

[AJerman]

Jesus, I'm really hoping American tyres are labelled differently to UK tyres. In the UK, the PSI on the tyre is the maximum the tyre is rated for. If you fill your tyre to this you're asking for trouble! You'll have greatly reduced grip and higher risk of blow-out. You should always fill the tyre to the car manufacturers stated guidelines +/- a couple of PSI for personal preference.

Nope, that's how they are labeled here, people just don't seem to understand what MAXIMUM tire pressure means. Not recommended, but maximum. Always use what it says on the door unless you specifically know otherwise (special circumstances).