Here is some basic math formula's i worked up to figure out roughly how much WHP would be free'd up when using a lighter wheel.

How about this:
For ease of calculations we will only calculate the drive wheels or rear wheels in this case.

We will use the OEM 19" performance M3 wheel
OEM 19" (Forged)
19x9.5 @ 29.2 lbs/ea

And the OEM BBS 19" ZCP lightweight M3 wheel
OEM ZCP (Flow-Formed Cast)
19x9.5 @ 24.2 lbs/ea

We will assume only the wheel weight will change.
Mathematically, there is the rotational inertia of a hoop about a central axis:
I=MR^2
M is the weight of the rim
R is the radius of the rim in meters
Which will be 0.2413m (taking it is a 19" rim).

Then there's the kinetic energy equation for a rolling body and since a rolling object has two types of energy: a rotational kinetic energy due to it's rotation about its center of mass (COM) and a translational kinetic energy due to translation of its COM, we will use:
K = 0.5*(I,com)*w^2 + 0.5*M*(V,com)^2
I is the rotational inertia.
W is the angular velocity.
M is the mass of the wheel
and V,com is the velocity of the wheel's COM

Now we can calculate the rotational inertias of the wheel.
(24.2lbs=10.9769kg)
I (OEM 19) = 13.2448kg*(0.2413m^2) = 0.7712 kgm^2
I (ZCP 19) = 10.9769kg*(0.2413m^2) = 0.6391 kgm^2

So we know the lesser mass equates to less inertia But we still don't know actual performance gains. To do so we will find the kinetic energy of the rolling body at a fixed speed. We will Use a steady 100mph. this will be represented by V,com in meters per second.(100mph=161km/h)
V,com = 44.7222 m/s

The conversions are all done to make sure we are working in consistent units of measure.

Now we calculate the angular velocity of the wheel.
w = V,com
R = 44.7222/0.2413 = 185.34 rad/s

Now that we have our information, we will put these into our equation for obtaining kinetic energy of a rolling body.

K, (OEM 19") = 0.5*(0.7712kgm^2)*(185.34rad/s)^2 + 0.5*(13.2448kg)*(44.7222m/s)^2 = 23460.40 J

K (ZCP 19") = 0.5*(0.6391kgm^2)*(185.34rad/s)^2 + 0.5*(10.9769kg)*(44.7222m/s)^2 = 22456.77 J

J is for Joule which is a unit for energy. However, we want to know HP not Joules. To convert a Joule to a Watt we need to divide 1 Joule with 1 second. Now from 1 watt to 1 hp, we use the common conversion:
1 watt = 0.00134102209 HP

Now we know the required HP to keep rotating the wheel at:
@ 100 MPH
ZCP 19" =30.11 HP*
OEM 19" =31.46 HP*

@ 20 MPH
ZCP 19" =17.03 HP*
OEM 19" =22.90 HP*
*per wheel

Now take the OEM 19" wheel HP required, minus the required HP for the ZCP 19" wheel and we have 5.87hp x2 equals a 11.74 Net WHP gain! @20mph and 2.7whp @100!

Quite respectable for only losing 10lbs total, And remember thats just changing the rear wheels!!!

Math doesn't always ruin the fun.
Hope this helps,

Wesley
4DrDb8GsR
wesmooy@gmail.com