Later Schwinn serial numbers / years made
During the 1980s,
(e.g. F8 = June 1988)system which
when it changed to the two-letter system.
Some Taiwanese (built by Giant) Schwinns have a date code (on the right rear axle hanger or on the bottom bracket shell) in the format 'Gmmyy'. The letter "G" followed by a two-digit month code (01 = January etc.) and two more digits for the production year (81 = 1981 etc.). An example would be 'G0181' and equates to "Giant, January, 1981".
On bikes made after 1982:
You can find a 4 digit serial number lightly stamped onto the headbadge.
The first three digits are the numerical day of the year,
and the last digit is the last digit of the year.
3654 is 12/31/84 or 94 or? you determine decade.
Generally the 4 digits on the headbadge breakdown as follows: First 3 are the date of build 1 thru 365, the 4th (last digit) is the last number in the year. So "0" probably stands for 1990. If you are not sure which decade the bike is from, check the parts and graphics on the bike to the pictures in the catalog pages.