A good 8 inch race converter will slip 3 to 5%, anything more is consider not good As already said many things contribute to the effeincy of the converters, mainly the size, maker and the application information furnished to the maker on the initial build I use a rule of thumb on converter slippage on counting on 8 to 10%, again depending on many things, a radial tire will change the converter slippage numbers due to no relative tire growth Maybe you should post the car weights, motor size and type,BB or SB, trans and converter size and brand, tire size in circumference and the intend RPM crossing the finish line and we can back into the gear ratio for you, also where your going to run the car , altitude and typical weather conditions like humdty, actual altitude, temperature and actual or corrected(specify which) barometric pressures.