Possible rear bumper mod
 

Possible rear bumper mod.

If one could remove trunk for spare wheel and bottom part of rear bumper and connect it with bottom cowering plate - it should be efficient and less costly than roof chopping. It would be ideal for sedans with long rear.

Can't have much rise before flow separates there, rendering the effort useless.

Of course you are right. 14-15 " wheel is about 20 in diameter with tyre. So we could have bottom partial kamback 20" length. It is more in length than more of top kamback..

That red curve is too steep and I'm quite sure wouldn't represent an aero improvement over stock.

Ok. That kind of slope in degree we would use ? 10-15 or less ?

4 degrees is what the number is around here for the bottom.

I think it's 10-15 on top.

That depends on the chassis shape: A fastback or sedan has lowest aerodrag (Cx) around 3°-4°, but a squareback (like the Fiat Idea you used as an example) is better off with a flat rear(*).
(*) If the car already has a Kammback, then it is no longer a typical squareback and a slight diffuser angle may be OK.

On the other hand, race and performance cars have more aerodrag Cx and less Cz (= more downforce) when the diffuser angle is greater than 5°.

Either way, getting rid of, or covering up, a spare that hangs down in the air flow is something to think seriously about.

That's usually where the muffler is hanging out, too.