Call your vehicle dealer's service department. Ask how many teeth are on the the spur gear and pinion gear of your differential. If for any reason you can't reach the service department for your car, try visiting GlobalSpec.com to find the data. Note, however, that the service department is the more reliable source, and a the web site may provide inaccurate or outdated information.
Find your car's transmission drive ratio in the owner's manual. This is usually located in the section of the manual on Vehicle Specifications under the section on the transmission.
Divide the number of spur gear teeth by the number of pinion gear teeth. Multiply this result by your transmission final drive ratio.