Tire Pressures

[FL335i]

Hey guys, what you running for tire pressures? My placard says 32psi on the front and 38psi on the rear. (cold). Can't understand why they would want more pressure in the rear when it supports less weight... Dealer put all 4 tires at 37 cold. I'm inclined to put all 4 tires at 38psi cold. Opinions? Experiences? Oh yea, 17" base wheels, Run Flat Continental tires.

[mryakan]

BMW recommends these tire pressures for handling purposes, mainly to dial in more understeer with the higher rear than front pressure. Understeer is a bit more friendly for lesser experienced drivers than oversteer.
You can put what you like up to just below the max the tires support, but then you have to yourself balance the pros/cons for handling, fuel economy, tire wear and comfort. The OEM specs are good for most people and a well balanced choice.
There are 4 OEM specs for each tire choice, 2 of them on the door jamb, one for normal load and the other for max load. Those are good for up to 100mph. Your manual has more specs for speeds >100mph.

[in transit]

I'd go with what BMW recommends.

[skierman64]

I believe the recommended tire pressures are also set for a vehicle at max placarded vehicle load. So the rear pressures are recommended to be higher to deal with 5 people in the car plus a trunk full of stuff up to the GVWR.

[FL335i]

I'll read up in the manul regarding this topic.

[FL335i]

Ok guys, read up in the manual. I'll be running the tires at 35F/42R cold psi. This is what they specify for factory weight loads and 100mph+. For the comfy under 100mph they call for 32/38. I'm just gonna go with the higher ratings for fuel economy reasons and high speed use with passengers & luggage.

[mryakan]

I believe with the higher tire pressure, you will lose some handling and comfort. Since in the US no one seems to be able to drive above 100mph for long distances at least, you may be better off with the 32/38. Also max load is really the car with 5 passengers and a trunk full of luggage, usually not the norm. So unless you are a taxi cab driver or are going on a long trip with the whole family stuffed in the car, I do not think there is a need to always have the higher pressures. You can always adjust them when needed, and should check the pressure every 2-4 weeks anyway.
But then if you do not drive aggressively or at the limit and do not feel uncomfortable with the ride, then there is no issue running the higher setting. It is all about tradeoffs and personal preference.

[FL335i]

Good point. I'll try both and see what they feel like and go from there.

[GunnerNell]

I have always run my tires a pound or two higher than the recommendations, for three reasons: 1) most manufacturers used to recommend lower pressures to make the car's ride softer; 2) harder tires are going to run cooler than softer tires, possibly enhancing the wear; and 3) harder tires are going to give you better gas mileage.

A lot of folks here used to flame me about that, asking whether I thought I knew better than the BMW engineers. No, no claims like that, and to be sure, I wouldn't lump BMW into the group that errs on the side of a softer ride. But there you are. It's a hard habit to break. I did notice my tires wearing in the center just a tiny bit more than on the sides, a sign of overinflation, but that was when I was running nearly 5 pounds more. I've ratcheted it down some now. But either way, I'll trade the better gas mileage for the thumpiness you get with harder tires any day.

[FL335i]

I do the same in my Wife's Honda and in my past Accord. Honda was like 28/32. I ran 35 all around. That's what I run in the wife's civic too. I seem to have very good luck with 35 all around in the honda. Flat wear, awsome MPG's. I might just try 38 all around in the bimmer they call for 32/38 and then 35/42 for 100MPH+. Funny thing is the dealer put them to 37 all around... leads me to believe a lot of bimmers are just getting one pressure all around. My Mother in law's Mini Cooper S calls for 38 cold all around. I'm really leaning toward 38 all around but will try all options.

The possible combos:

32/38 Up to 100mph
35/42 100mph +
35/38
38/38

Has to be one of them.

[GunnerNell]

I believe my E46 recommended 32/38. I ran 34/40 or 35/41 most of the time. The really important thing is to check and fill them when they're cold. Running even as little as half a mile can heat them up to where you won't get an accurate reading. A good digital pressure gauge and an air tank are your friends.

[mryakan]

I have always argued that you can fill tires when hot as long as you measure them when cold. Since the volume of air in the tires should practically stay the same as long as there are no leaks, then if you measure X psi when cold and then Y psi when hot, then take out the difference (Y-X) and add it to the desired cold pressure and then fill accordingly. e.g. assume you measure 30 psi when cold and then drive to a nearby gas station and measure 32 psi when the tires got hot. If your desired pressure is 35 cold for example, then you fill to 37 since the difference between cold and hot in your case was 2 extra psi. Pretty simple math that saves you from buying an air tank or compressor, or living next to a gas station or tire shop. You can always check the cold pressure again the next morning, it will not kill you to have a couple of PSI extra for one day anyway.

[FL335i]

I have a 10 gallon air compressor at home along with some high end large dial gauges. I check our tires every other sat morning while the coffee is brewing. Cars are garaged, so the tires are cold.

[GunnerNell]

I have always argued that you can fill tires when hot as long as you measure them when cold. Since the volume of air in the tires should practically stay the same as long as there are no leaks, then if you measure X psi when cold and then Y psi when hot, then take out the difference (Y-X) and add it to the desired cold pressure and then fill accordingly. e.g. assume you measure 30 psi when cold and then drive to a nearby gas station and measure 32 psi when the tires got hot. If your desired pressure is 35 cold for example, then you fill to 37 since the difference between cold and hot in your case was 2 extra psi. Pretty simple math that saves you from buying an air tank or compressor, or living next to a gas station or tire shop. You can always check the cold pressure again the next morning, it will not kill you to have a couple of PSI extra for one day anyway.

I agree for the most part, except if you're OCD like I am. I always wonder how much that 2 or 3 pounds of cold air pressure changes/adds when it hits the hot air in the tire, and whether different tires might heat up to different levels, thereby causing different changes in the changes. (Yeah, it's tough being me.) Mostly, it annoys me to have to check the tires two days in a row when I do it this way to ensure I have the desired pressure all around.

FL335i, I think "cold" means whatever the ambient temperature is, but I suspect you get the most accurate readings when the car is garaged. I usually try to add a little fudge factor if one side of the car is exposed to the sun and the other side is in the shade.

So then how about this -- In the winter, should I reduce the measured pressures, knowing that the pressure at 32 degrees is going to be much greater when the tires heat up than it would be if I measured them at 70 degrees? IOW, if I want 34 psi, should I fill them to 32 psi at 32 degrees to allow for more expansion on the road, or ... ? Like I said, it isn't easy being me (although no, I don't lose a lot of sleep over this ;)).

[FL335i]

My garage has no windows so there is no sun factor. All 8 tires are even. (2 cars). In my wife's civic, 35 seems perfect. Anything higher really makes the ride rougher and there is not noticable MPG gain. HONDA calls for 32 all around. So I bumped it +3psi all around. Even my honda liked 35 all around. I started at 50psi (tire's max) and worked my way down with marking paint on the tread and ride comfort/handling. 35 is where I kept coming back to in both Hondas. I'll play around with my 335i... See what I end up with.

[mryakan]

My garage has no windows so there is no sun factor. All 8 tires are even. (2 cars). In my wife's civic, 35 seems perfect. Anything higher really makes the ride rougher and there is not noticable MPG gain. HONDA calls for 32 all around. So I bumped it +3psi all around. Even my honda liked 35 all around. I started at 50psi (tire's max) and worked my way down with marking paint on the tread and ride comfort/handling. 35 is where I kept coming back to in both Hondas. I'll play around with my 335i... See what I end up with.
Don't forget that the weight that the tires carry also has a lot to do with what pressure they should have (that is why higher pressure is recommended for a fully loaded car). I suspect that your Honda is somewhat lighter than your Bimmer. Something to ponder upon.

[FL335i]

35/38 seems perfect. :) Going to let it fly like that and monitor wear.