Wind turbine impact on cars fuel consumption I have question that I still can't resolve. How much cars fuel consumption rises(l/100km) if I attach 1 kW wind generator to cars body? How can I calculate that and what do I need to know? 
Respectfully,
Ivars
 A: As pointed out by the comments, it will cost more energy than what the turbine will generate... if the car is riding in calm weather without wind.
But what if the car is driving at a velocity $v$ while the wind is blowing at a velocity $u$? The relative wind speed will be $w=u-v$. According to Betz's law, a wind turbine with a swept area $S$ and an air density $\rho$ has a power yield 
$$P=\eta\rho |w|^3 S/2, $$
where $\eta_0=0.59$ is the theoretical Betz limit. This doesn't mean that the other 41% are lost as heat; it's just that the air will simply flow around the turbine if you try to extract more energy. In the Betz analysis, there are no friction losses. A real wind turbine, operating at its optimal wind speed, may have an efficiency $\eta=0.44$. In the following analysis, I will assume an ideal wind turbine with $\eta=\eta_0$.
The Betz analysis also implies that the force (not torque) exerted on the turbine is $F=\eta\rho w^2 S \mathrm{sgn}(w)/2 $, where sgn(x) is +/-1 depending on the sign. However, the work done by the car engine (power $P_{\mathrm{en}}$) is relative to the road surface, not relative to the wind speed, i.e., $P_{\mathrm{en}}=-Fv$. The ratio is
$$r={P\over P_{\mathrm{en}}} = -\frac{w}{v} = \frac{v-u}{v} = 1-\frac uv.$$
There are a lot of scenarios depending on the magnitude and sign of $u$ (assume that the car speed is always positive, $v>0$):


*

*$u=0$: the power delivered by the wind turbine is equal to the extra power needed at the wheels. For a wind turbine with $\eta<\eta_0$, you need more power at the wheels than you get from the wind turbine.

*$u>0, u<v$: the wind comes from the back; car is going faster than the wind speed. Result: $0<r<1$. The engine power is positive (using fuel/battery) and the electricity production of the turbine is less than what the engine needs. The extra energy goes into accelerating the air mass around the car.

*$u>0, u>v$: the wind comes from the back and the car is going slower than the wind. Result: $r<0$. The engine receives power through the wheels. A gas-powered car would wear out the brake pads. An electrc car would recharge the batteries both from the engine and from the electrical power coming from the wind turbine.

*$u<0$: the wind comes from ahead. For a normal vehicle, this would be a lot of extra air drag. Here, $r>0$: the extra engine power to overcome the drag of the wind turbine is less than the power delivered by the wind turbine!
One question one may ask is how much advantage you get with a moving wind turbine compared to a stationary one. Consider a case with wind speed $u<0$. Let's define the constant $\alpha\equiv \eta\rho S/2$. If the car is not moving, then the generated power is $P_0=-\alpha u^3$. If the car is moving at positive velocity $v$, then the generated power is $P=\alpha(v-u)^3$. The engine power necessary to achieve a velocity $v$ is $P_{\mathrm{en}}=\alpha(v-u)^2v$. Now, is the extra engine power is more or less than the increase of generated power? We can write:
$$
P_{\mathrm{net}}
= P - P_0 - P_{\mathrm{en}}
= \alpha(2vu^2 - v^2u).
$$
The formula is for $u<0, v>0$. It may or may not be correct for other signs of $u$ and $v$. It turns out that there is a net energy gain: for every watt of engine power to the wheels, you get more than a watt back from the turbine! If you reach the end of the stretch of the road, you'll have to turn around. You'd better fold the wind turbine, then gently drive back to the beginning of the road, and start over again. For $|v|\ll|u|$, the ratio is 2 to 1: you net two watts for every watt of extra engine power; alternatively, the wind turbine generates three watts extra for every watt of engine power. The incremental gain drops as $v$ increases; it's probably not worth the trouble for $v>2|u|$ or so.
Finally, as for the question of how much the fuel consumption is: as you can see above, it could be zero, depending on the wind speed. It also depends on the size $S$ of the wind turbine. If we assume that $S=3~\mathrm{m^2}$ is about the maximum that will fit on a car, $\rho=1.3~\mathrm{kg/m^3}$, then the power produced by the wind turbine is $P=1.15~\mathrm{[kg/m]}w^3$, with $w$ the relative wind speed in m/s. If we set $u=0$ (no wind), and the car drives 50 km/h ($v=13.9$ m/s), the power delivered by the wind turbine is 3 kW. For a combustion engine (20% efficiency), that would be around 3 L/100 km of added fuel consumption.
