Firecalc allows to choose a portfolio mixing various asset classes (e.g. S&P 500, government LT bonds, corporate bonds, small cap, small/large value, etc).
This means that there is an underlying database of returns on those various asset classes, which has been backtracked to 1927, either exactly or at least a decent approximation.
It isn't easy to find such historical data before 1972+ (besides S&P 500 and T-Bonds)... Been trying for a little while!
Any way the Firecalc designers could give us access to such historical returns per asset class, in an Excel sheet or whatever tabular format?