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My experience with newer BMWs is that they all read 5% high. So the faster you go, the higher the delta. This is how I mentally compute the delta at any speed:
1. Divide your speed by ten. Easy to do. 40mph -> 4
2. Divide that value by 2. Easy to do. 4 -> 2.
So at 40mph, it will read 42.
At 75: 75 -> 7.5 -> about 3.5 So it will read 78-79mph at 75 mph.
I did very well on a Time-Speed-Distance rally using a late-model BMW with a HUD and mentally performing the math on my speed using the 5% value.
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