I just put Dunlop FM901 tires on my stock AP1 rims using the traditional 205/55/16 and 225/50/16 staggered tire sizes. However, I am not certain the the wheels were inflated to the correct pressure, because the car seems very eager to "wander" when on the highway. I'd like to perform a few tests myself to ensure that everything is in good shape, and would like to know how to determine the proper pressure to inflate my tires to.

Can anyone offer some suggestions/advice on how to best determine the proper pressure?

Thanks in advance.

 

You should run the air pressure that is listed on the door jamb. If you want you can go a few lbs. higher and get a little better handling.

 

Hey Jim, I was wondering if this way of determining tire pressure is true...

For the rear you take the weight of the car (exam: 2800) and take away the last 2 digits and that should be your rear tire pressure.

Rear= 28psi

and then to get your front you add 2-4psi to that.

 

You may be suffering from not only tire pressure considerations but from having selected the wrong size tires for the rear. A non-OEM tire should be sized 245/45/16 for the rear. Read to the stickied thread at the top of forum to see why.
Your car may be "wandering" because you don't have the correct "actual" tire tread width contact patch on the road. Cars that go with staggered tire sizing need a particular width differential between front and rear to reduce the tendency to wander.

 

Yeah... I just read that, actually. Unfortunately, I "inherited" these tires/rims from another user, who had brand new Dunlop FM 901's on the rear. It seems such a waste to let those go to waste. I seem to be cursed with bad wheels for this car though... because I bought the car with aftermarket motegi 17' rims with 225's on all 4's. At least I'm getting closer to the 'correct' stagger.

Do you think I'm going to have any kind of significant issues if I just let these stay on the rears? I'm willing to scrap the 225's if necessary, but I thought I might give them a chance.

You're word is my command. What should I do?

 

Any thoughts? Anyone?

 

Well, you shouldn't just take my word for it. Others may have opinions on this as well.
I believe you will be OK to run these tires till you need them replaced, at which time, you can get the proper sizing or go with the proper sizing on the 17s. You already know that your car tends to wander at speed, so this knowledge will help you to be prepared to correct when necessary. I would suggest that you experiment with the tire pressures in the rear. The fronts are good sizing so just go with what's on the door jamb (for now). You can reduce the rears by 2 to 4 psi, go out for a drive and see if that made any difference. If it gets worse, then go 2 to 4 psi over what's stamped on the door jamb and test again. This will tell you if you are going in the right direction. You may not get rid of all the wandering and you will always have less stick in the tail on lateral manouvers, so beware of this and you'll be fine.
Then, if you feel it worthwhile, do the same experiment with the fronts.

 

Thank you for the advice. I love experiments! I'll be sure to report what I find, as time passes.

 

All pressures should be checked with cold tires. Tire pressures increase up to 4-5 psi after the car is driven for several miles. After one run, you should wait for few hours, then add/remove air for next experiment. If you do not like to wait that long, you should add/remove air with the correct amount, regardless of tire pressures that you check right after the first run.
Example, first run with 30 psi (2 psi below recommended of 32 psi), remove 2 psi for second test regardless of air pressure after first test, this will give you 28 psi cold. Add 6 psi after second run to test 34 psi cold, add another 2 psi for the next test to represent 36 psi cold.

 

An excellent point!

I'll take this into consideration as well.
Thank you.