Naumoff,
Can't believe it... Prince Fredrick, Md. Eh? Bout fell outta my chair when I saw your inquiry and where you are from... me too for part of my life. People usually say to me "where is that?". Its where my father finished his life out, and where I spent many summers growing up. My cousin and some old friends live there. Sort of felt homesick for a second when I saw your post. Many many memories on the Pautuxent and along the Bay.
Ya ever met Eddie Trail who owned the Trail's Amoco (great mechanic) there for many years? He taught my Father and me many things about building performance stuff... lots of boat fun and racing back then. Eddie now has a private shop and the last I saw him he was still doing some high-end musclecar stuff for people, conversions, etc... He could tell ya a thing or two about gear ratios.
Just wanted to say Hello.
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IMO: Extra light weight performance cars like Cobras don't need low gears (torque multiplication factor) on the street. It does depend on what motor you are building. If its a torque motor, go a bit "Taller" on the axle ratio and you'll see what the phrase "takes off like a slingshot" means. With a 4-speed TL, 9" equipped cars seem to do well with 3.25:1 axle ratio and Jag/Dana rears at 3.31:1, and even better with a 5-speed.
Good Luck!