Longest lasting series?

There are four semifinalists.

Nash Ambassador
Olds 98
Chrysler New Yorker
Buick Roadmaster

All other names were clearly shorter.

Criteria:

1. Must remain in continuous use except WW2 when all cars except Jeep were interrupted.

2. Must remain on the same step of the ladder, never descending the stack like BelAir or Fairlane.

Ambassador started in 1927 and continued until 1974. It was always the top Nash or Rambler. It wasn’t always a distinct chassis or size; from ’27 to ’31 and then from ’62 to ’64 it was just a trim level. Total 48 years without a break.

Roadmaster ran from 1936 to 1958 without a break, then resumed from 1991 to 1996. It breaks continuity, and it was also pushed down the stack in some years by the Limited.

So the finalists are New Yorker and 98, both around 56 years depending on how you mark the start.

New Yorker ran from 1939 to 1996 without a break, but it was pushed down by Imperial at some points, and also dropped below the Saratoga once or twice in Chrysler’s perpetually confused marketing scheme.

98 started in 1940 as a distinct label and ended in 1996. It was always the top level, while the lower levels shifted around. (Olds had two-digit models before 1940 but wasn’t treating them as series names.)

If we omit the second criterion, New Yorker barely wins on longevity, but if we follow all the rules, 98 wins.

It’s interesting that all three of the names used by GM and Chrysler ended in 1996. There wasn’t a major transition in the ’90s. No gas crisis, no sudden change of size or economic conditions.