If you have an iPhone, or iPod Touch, go to the App Store and download GearIt. It is the best piece of software for gear ratio calculation ever devised!! It costs 59p in Europe, probably 99c in US.
GearIt works with any car - TC, Off Road, Pan, Rallycross, Gas Track - and allows any available range of pinions and spur gears you can think of. It is independent of pitch (DP) of gears used, and independent of metric or imperial units. You get automatic updates, and you can add new cars at any time (internal ratios)
So clever, so simple... so buy one! HTH