In this paper we review the characterization of the sustainable rate of return of an earnings-related pension system with pay-as-you-go financing. We show that current proxies for the sustainable rate, including the Swedish “gyroscope”, are not stable and propose an alternative measure that depends on the growth of the buffer-stock and the pay-as-you-go asset. Using a simple one-sector macroeconomic model that embeds a notional account pension system we test how the different proxies perform in the presence of various macroeconomic and demographic shocks. We find that the new formula proposed in this paper is the most stable. It avoids the accumulation of assets without bound (which penalizes workers) while always ensuring a positive buffer fund.