What other reason could there be? I did nothing aside from changing the tire pressure.

I drove to my brothers, increased tire pressure at his house and drove home. Road conditions were the same both ways. The only varied factor was tire pressure. Steering was better, braking was better, acceleration was better and understeer was greatly reduced. So what other factor could there be?

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Current Bimmer
2004 X5 4.4i Sterling Grey Sport/Premium

Past Bimmers
1991 318I Alpine White
1995 740I Alpine White
1991 525I Schwartz
1998 323IS Scwartz
2004 330CI Cabrio Titanium Silver Metallic
1995 540I Schwartz
2000 Z4 3.0 Titanium Silver
2000 330ci Coupe Titanium Silver