2008 535i 18.3 mpg???
03-28-2010, 08:53 PM
#31
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One of the forces at play here is called Moment of Inertia. You can have a 17" 22 lbs wheel and a 20" 20 lbs wheel. The 20" wheel requires more power to accelerate due to Moment of Inertia even though it's lighter. Different tires and how wide they are also increases/decreases rolling resistance and Moment of Inertia.
Also, im willing to bet that an inch more tire width and 20 lbs of weight (assuming 5lbs more for each wheel) isn't going to make 2mpg difference.
If the OPs overall diameter is different than original and he hasn't had his speedo calibrated, the mileage could appear to be less... A big mileage difference is definitely seen when going from one diameter tire to a much larger tire, like if you went from 30" tire to 36" or something.
Just my .02...
03-29-2010, 01:30 AM
#32
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My Ride: 2006 550i, 1984 Mercedes Gelandewagen
One of the forces at play here is called Moment of Inertia. You can have a 17" 22 lbs wheel and a 20" 20 lbs wheel. The 20" wheel requires more power to accelerate due to Moment of Inertia even though it's lighter. Different tires and how wide they are also increases/decreases rolling resistance and Moment of Inertia.
Wheel 17" at 22 lbs + tire 26 lbs (Yokohama ADVAN Sport 225/50R17) = 48 lbs; r=12.9in
Wheel 20" at 20 lbs + tire 24 lbs (Hankook Ventus V12 235/30R20) = 44 lbs; r=12.8in
You are correct about one thing, the larger wheel and TIRE are lighter.
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