Phu... What can I say?
First of all...I think the sigs with cars in it are the hardest to give a good overall feeling.
The cars don't pop out.
The text...ohh dear, the text
Everything you've written on your sigs is hard to read or choppy... We don't want that now do we?
ok, the first sig:
+ Background seems ok
- Font is choppy
- Stroke around the car?
- Car layer seems to be on difference or overlay, when you want it to jump out, never use overlay or difference!
- You tried to make the car softer by adding blur? (It shows at the stroking around it)
score: 4/10
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second sig:
=> same comment as 1st sig
score: 4.5/10
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third sig:
+ Background seems nice
+ Good border (without the stroke)
- No overlay or difference
- Don't stroke the softened border
score: 5.5/10
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fourth sig:
+ Border
- Car painted in photoshop?
- What does it say on the top?
score: 5/10
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Fifth sig:
+ Background
+ Render color
+ Stroke around render
- The text Jay-Z (Stroke it with another color. Black for example)
score: 6.5/10
The reason why you can't find a tut about it is because it's so hard to create a good signature with a car in it. I've only seen 2 guys who performed a good job at it.
One last hint: Work on your text & EXPERIMENT!
Do different things, try something else. Your car sigs are all pretty much look-a-likes. You've done the same things to them. I'm not saying that the 'same' is bad, but you've got to have something good to do the same.
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PS: This is my opinion and I hope you will respect them as much as I respect your opinion.