Tire pressure

[Willymammoth1]

I am installing 225 40 18's on 8.5 inch rims on front and 255 35 18's on 9.5 inch rims on the rear of my otherwise stock 2001 325i. I am assuming I won't be running the same air pressure that I was with stock. How do I know I have the correct pressure?

[TJ710]

first of all are you sure those tires will fit in the rear? i thought 245 was the widest you could go on a standard 3 series

[Hallen00]

You are going to want to run higher pressures, but it really depends on the tires what those pressures are. You will want the pressure high to help protect the wheels because they are so low profile.

Your ride is going to be pretty harsh along with a decrease in handling capability. That is the price you pay for style.

[TJ710]

i would not want to run that size tire on my stock 325i. I went from the stock 205's to 225's all the way around and that suits me perfectly, very spirited driver. i would only go to 255 if i had a substantial increase in horsepower. that staggered set up might promote understeer as well...

[Critter7r]

You are going to want to run higher pressures, but it really depends on the tires what those pressures are. You will want the pressure high to help protect the wheels because they are so low profile.

Your ride is going to be pretty harsh along with a decrease in handling capability. That is the price you pay for style.


Your vehicle will technically require a lower tire pressure since you will have more area of the tire in contact with the ground. (More area touching the ground means you need less air pressure per unit of area to support the same weight).

Running the pressure higher won't necessarily protect your rims either, it'll just transfer the energy of the bump through the tire directly to the rim, and depending on the severity of the bump, could damage the rim. That being said, a lower pressure won't necessarily save your rim either, because if it's too low, the tire will deflect and the rim will get damaged anyhow. So it's best to just run about the same pressure that the sticker in the door says to run for 17" tires and realize that 18's are subject to being damaged due to the low profile, regardless of the pressure you run.

[bigdog68]

I am about to put some 18's on my car but I am going to run 225/40s all around to keep understeer at a minimum and maximize performance, as far a tire pressure I will keep the same lbs from the 17s I run now

[fun2drive]

If you want to know how to adjust the tire pressure for even tire wear assuming no camber issues then use a piece of chalk and mark a line across each tire perpendicular to the rotation of the tire. Do this someplace where you can roll the car straight. A mall parking lot for example when not busy or a school parking lot after school. Drive the car straight for a short distance and get out and check the wear mark on the tire. If it is wearing equally all the way across then you have the right pressure for street driving.
If in the middle then you have too much pressure, if on the outside edges too little.
This disregards totally the effects you want for handling such as understeer and oversteer using tire pressure and is suitable for only getting the right pressure for even tire wear.

This is how over the road trucks gage what pressures to run against loads and it works fine for our applications too.
If you want to know how to set your car up to handle then go to the SCCA and they have a good page or more on tire pressure set ups for what you want to do.
Hope this helps some...

[The HACK]

first of all are you sure those tires will fit in the rear? i thought 245 was the widest you could go on a standard 3 series

E46 330i with sports package comes with 255s in the rear. There's no practical difference between the 330i and 325i chassis when it comes to wheel clearance and suspension parts/geometry.

I don't see why 255s won't fit on a 325i. I'd question the practicality.