The lighter the wheel better the fuel economy. So if the 21"weels+tire is less in weight than the 20" you will get better fuel economy.

I am not sure if the speed will change because you will probably compensate for the increased wheel diameter with a tire with lower profile. Technically, if the wheel+tire diameter is bigger then the speed will be higher (your speedo and odometer will be shot though and will show lower speed and distance respectively)

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2008 E70 X5 3.0si
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2019 G30 530e
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2018 F83 M4
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2009 E93 335i (Sold)
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E36 328is GoKart (Sold)
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