To put it in perspective. Marco Pantani's climb up L'Alpe de Huez averaged 23kph, or about 14 mph. Up a 9 mile climb with an average grade of 8%.
If you want to keep turning 90rpm at 14mph you need a 39/21. Given that there are sections on that climb for example that go 11-12%,and even he couldn't maintain 14mph, you'd need a 23 or even a 25 if you wanted keep spinning.