Avoid the black shirt, solely white letters idea, as anyone can do that and there's nothing that comes close to making your product definitive.
A more mature design needs to be implemented, but those you listed almost seem too conservative to me. It's really tough to get the right mix of design and conservation.
Depending on which route you want to take with the company, it might be a consideration to put some time in and write a back story to a novelistic speed shop, then sell products similar to this:
http://i.ebayimg.com/t/MENS-BIKER-MECHANIC-WORKSHIRT-Rusty-Nuts-Auto-Shop-USED-PARTS-M-XL-2X-3X-4X-5X-/00/s/MTYwMFgxMjAw/$(KGrHqRHJE8FD(Hf(C7)BQ2gk+WV!g~~60_57.JPG
Or you could just go with badass, simple designs that catch people's eyes:
http://i.ebayimg.com/t/PANDA-IMPACT-MECHANIC-T-SHIRT-MAZDA-HONDA-NISSAN-TOYOTA-DRIFTING-TURBO-Cute-/00/s/NzQ3WDg2NA==/z/xrgAAMXQRDxREG1Q/$(KGrHqN,!hcFC80Il!t9BREG1PfQUg~~60_57.JPG
http://i.ebayimg.com/t/BATTLE-FOR-BOOST-TSHIRT-WANKEL-TURBO-ROTARY-ENGINE-MAZDA-RX7-RX8-RX2-12A-13B-20B-/00/s/NTAwWDUwMA==/z/xbMAAOxyq5NRC0aE/$T2eC16R,!y0E9s2S6c10BRC0,!plb!~~60_12.JPG
http://i.ebayimg.com/t/1987-Porsche-944-Turbo-Grey-T-shirt-S-M-L-XL-XXL-/00/s/ODAwWDgwMA==/z/h7cAAOxyOBJRDY3U/$T2eC16J,!)sE9swm(wBWBRDY3UHEKQ~~60_3.JPG
One of the biggest things to remember is association is key: people LOVE to be walking billboards for just about anything. If you can come up with a product of your own that fulfills people's need to advertise, most of the battle's already won.