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my M3 , with 19's and the correct aspect ratio as per BMW was reading 7kmh faster then actual speed. My Z4 with factory 19" rims and factory tires reads 5kmh faster than actual speed and my 325Ci with factory 17's is dead on the speed. +/- 1kmh (all measured with two different GPS units. I started checking when everyone passed me on the highway when my cruise was set at 120, the regular speed of traffic on that road).
I drive using GPS as my speed indicator, not the speedo. I like accuracy not approximation.
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Member ZSCCA 2009 Z4 s35i with Dinan Stage II software, Sport and Exec packages, 19" 296 rims , a stubby and mud flaps
'03 Z4 3.0i Sterling Grey (Hers)
'01 Z3 3.0i Sapphire Black, M sport package (Mine)
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