Can you figure the difference in percentage of the old rear tires compared to the old front tires? Example, old rear tires are 48'' dia. old front tires are 30' dia. Front tires are 62.5 % as big as rear tires. New rear tires 50''. New front tires would be 31.25" If you want more or less lead or lag in the front tires, add or subtract the needed percentage. On stock car tires we would use the circumference of the tire because that is a more accurate measurement.