Yea that comes close to the idea I had. I wouldn't give up so fast with that. We should discuss this a bit more. Flynfrog basically might be right than you get a bit more drag by changing the speed of the air. But we try to regain that with low pressure zones. I would also say that’s more the reduced cross section who adds drag than changing the speed of air and reducing the cross section isn't even necessary because it was already reduced. Keep in mind that your Co2 canister also reduces cross section of the tunnel so a complete straight tunnel as flynfrog drawed it can’t be good.
Whenever you change the velocity of a fluid you incur a penalty - drag. Having changed the direction of the flow where necessary - in your case around the CO2 canister housing and the wheels you want to transition the geometry behind the object smoothly and gradually along a streamline (i.e., like the trailing edge of an airfoil).
Even with your old design you changed speed and direction of the air because you had to guide it around the wheels. What you basically want is high pressure behind your wheels and low pressure in front of them. So we apply Bernoulli’s Law who says that high speed causes low pressure and low speed causes high pressure. This could reduce the drag you already had even when it increases it on the first glance. Moreover your cars profile got more streamline and therefore reduces the possibility of a stall especially at the rear. Remember you want smooth transitions you don’t want a sudden change or a edge.
Look at the results of your simulation. You have high velocity at the rear in the channel of the car(red arrows).
And therefore you have low pressure behind your car what causes also drag (blue area).
Maybe its not even necessary to decrease the cross section at the smallest point further but the expanding rear section looks definitely better than your old design. Best thing would be to test it with a simulation. I would be interested in seeing the results. because this is the best way to learn for all of us.
A step back:
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You should also think about what you want to do behind your front wheels. From your simulation it’s visible that there is some air flowing from the inside of the tunnel to the outside behind the wheel. I tried to explain a few times that you should care about that area.
