SCHWINN CHOPPER SUZUKI FA 50 front sprocket location- (rear rim on motor is converted to a sprocket) 13 tooth. Rear sprocket location- (rear back tire on the schwinn 20 inch rim) 24 tooth. Mathmatic back to front- rs 24 x fs-13= 15.5mph I wish to re gear the (REAR ONLY) SO what tooth mathmatic do I need as shown to equal listed. Rs? x fs13= 21-25mph?