hi i wanna do a car cake the dimensions are 20 cm length and 8 cm width
i have a 24cm round cake pan and a rectangular pan with dimensions 27*17cm
which is best to use for the cake
ps. im gonna be bakin' a chiffon cake
Seems to me (and with half a lifetime between me and my last science class, I had to do the actual calculations in order to visualize a pan with Metric dimensions) that the "loaf pan" (English dimensions roughly 6.7" x 10.6") would be would almost certainly be the better choice for sculpting a car with minimum waste. Also, your projected finished size of 20 x 8 cm (around 3" x 8" English) seems like it would not produce a whole lot of servings; depending on how high your loaf is when it comes out of the oven, you might want to scale it up to fit the available cake.
Best of luck; I've never attempted to sculpt a cake, myself.