Harsh ride with new rims/tires

[mech9T8]

I bought a set of used M68 wheels and tires (17" for those that are not familiar with the style). Tires are Kuhmo MX (235F/255R).
I had 16" before so I knew that the ride would be a little firmer but it actually feels quite a bit firmer. I wondering if it's because the side walls on the Kuhmos might be stiffer as well.
Any thoughts?

[Mystikal]

The sidewalls are pretty stiff on MX's, but check the pressures too.

[mech9T8]

I set the pressure to 32 psi front and rear to start.
I don't think I should go any lower than this.

[Dr. Flyview]

A little offtopic, but I set my pressures kind of high (I think 33 and 36) but it was cold and now they're probably at 35/38. Gives me less traction and better fuel mileage right? I was surprised it started slipping at ~75km/h on the wilson heights north ramp off allen rd.

[Mystikal]

Originally Posted by mech9T8

I set the pressure to 32 psi front and rear to start.
I don't think I should go any lower than this.

No, that's about as low as you would want to run. It is just the tires then, unfortunately. You'll get used to it quickly.

[Mystikal]

Originally Posted by Flav_cool

A little offtopic, but I set my pressures kind of high (I think 33 and 36) but it was cold and now they're probably at 35/38. Gives me less traction and better fuel mileage right? I was surprised it started slipping at ~75km/h on the wilson heights north ramp off allen rd.

35/38 shouldn't be enough to make any real difference in performance, at anything under 10/10ths track/autox driving.

BTW, IIRC you're running 235/40/17s? The minimum pressure for those actually is 33/36, as I just set mine to that as per the door plaque.

[mech9T8]

I have 235/45/17 front and 255/40/17 rear.
What pressure do you recommend?

[Dr. Flyview]

Originally Posted by Mystikal

BTW, IIRC you're running 235/40/17s? The minimum pressure for those actually is 33/36, as I just set mine to that as per the door plaque.

Previous owner put in 225/45/17s. I don't know why everyone is getting xxx/45s/17s ...they're too tall!!

Hmm when I looked at my door panel ratings, they were so low...and I don't quite understand what it's trying to say with load and such.

[Mickey Knox]

flav, that means if you load up your car with ppl and luggage you need to increase the tire pressure accordingly

[Mystikal]

Originally Posted by mech9T8

I have 235/45/17 front and 255/40/17 rear.
What pressure do you recommend?

Those are huge tires. When you have to replace them, go to 225/45/17 front and 245/40/17 rear (or just 235/40/17 all around if you don't care to stagger them).

Anyways, I'd go with anything in the mid 30s, with like 32psi as the minimum. Slightly higher in the back.

[Mystikal]

Originally Posted by Flav_cool

Previous owner put in 225/45/17s. I don't know why everyone is getting xxx/45s/17s ...they're too tall!!

Hmm when I looked at my door panel ratings, they were so low...and I don't quite understand what it's trying to say with load and such.

I know what you mean, I think 235/40 looks so much better and is a lot easier to fit.

Stick to the door panel ratings, seriously. Unless you're at a track/autox event, those pressures are actually good to run. Of course it depends on the tire too, for example if the stock tires are UHP summers and you put on all-seasons with soft sidewalls you're going to want to up the pressure slightly.

[mech9T8]

Thanks Jay.
I know the tires are slightly oversized. I'm just happy they fit.
I'll put the proper size on when they are done.