Question: I'm looking for data on the average tariff levels of various countries from 1962-1989. Any version of the average tariff (weighted average) would be fine.
Answer: For any statistics question, the Library's Database page for Statistics and Numeric Data is a great place to start. From there, SourceOECD, the UN Common Database (UNCDB) (replaced in Feb. 2008 by a new site, UNdata), and the World Bank's World Development Indicators are good sources for international statistics.
In this particular case, however, you'll need to go outside of Stanford. The United Nations Conference on Trade and Development (UNCTAD) has kept trade statistics since 1964. Recently, UNCTAD, the World Bank, UN Statistics Division, and the World Trade Organization (WTO) combined resources to build the World Integrated Trade Solution (WITS), which gives access to the major trade and tariffs data compilations: the COMTRADE database (maintained by the UNSD); the "TRade Analysis and INformation System (TRAINS) (maintained by UNCTAD); the IDB and CTS databases (maintained by the WTO).
You can use TRAINS to get average tariff statistics. It provides online access to indicators of Trade Control Measures (Tariff, Para-tariff and Non-tariff measures), as well as imports by suppliers for over 150 countries. Registration is free at wits.worldbank.org. There is a registration link here.
TRAINS goes back to 1988. The World Bank has a page devoted to data on trade and import barriers. There's a helpful -- though incomplete -- dataset called "Trends in average applied tariff rates in developing and industrial countries, 1981-2005."
For data prior to the 1980s, search the journal and documents literature and/or do your own calculations for average tariffs. Worldwide Political Science Abstracts, EconLit, and the World Bank e-Library are good sources for journal articles about international trade.
Also check the following documents for possible leads and data tables:
Lastly, the library has a subscription to the International Customs Journal, published by the International Customs Tariffs Bureau (ICTB). This journal lists provisions of each country's customs tariff law and has detailed lists of items (steel, textiles, machinery, arms, etc.) and the tariff charged for each item, going back to 1891 in microfilm, print, and CDROM. More recently (2000-present), the ICTB has made that information accessible online here.