Currently I have 215/50-17 on the front and 245/45-17 on the rear of my S2000, the OEM sizes are 215/45-17 and 245/40-17. The diameter is about 3.5% larger and load index increase from 91 to 95 for the rear with the same W speed.
The placard on the door has 32 PSI for both front and rear, since the diameter increases 3.5% for both front and rear, the same pressure is not applicable for larger diameter, I have both front and rear at 32 PSI the performance/handling was terrible, when I went home I looked at the dust on the tires, the front had dust on 3/4 of the outside tread but the rear had dust only 1/4 of the outside tread, so I reduced the rear pressure to 30 PSI then 28 PSI then 26 PSI. The car is now very stable at highway speed(more than 70-75 MPH) and lateral acceleration is very good too. When I drove slowly on dirt road the dirt covered 3/4 of the outside treads which is much more than 1/4 when it had 32 PSI. I will try 24 PSI to see if performance/handling will improve or not.
Lesson learnt: When larger diameter(or wider) tires are used you need to adjust tire pressure for your particular vehicle, the lighter the vehicle the lower pressure may be needed, pressure on placard doesn't apply here.
The placard on the door has 32 PSI for both front and rear, since the diameter increases 3.5% for both front and rear, the same pressure is not applicable for larger diameter, I have both front and rear at 32 PSI the performance/handling was terrible, when I went home I looked at the dust on the tires, the front had dust on 3/4 of the outside tread but the rear had dust only 1/4 of the outside tread, so I reduced the rear pressure to 30 PSI then 28 PSI then 26 PSI. The car is now very stable at highway speed(more than 70-75 MPH) and lateral acceleration is very good too. When I drove slowly on dirt road the dirt covered 3/4 of the outside treads which is much more than 1/4 when it had 32 PSI. I will try 24 PSI to see if performance/handling will improve or not.
Lesson learnt: When larger diameter(or wider) tires are used you need to adjust tire pressure for your particular vehicle, the lighter the vehicle the lower pressure may be needed, pressure on placard doesn't apply here.