I actually did some analysis of my SS withdraw scenario analysis. This is for my data only and not for DW, yet.
What I did is first look look at all the SS taxes I paid over the years and deflated them into 2013 dollars.
Then I projected what SS taxes in 2013 dollars I will pay year by year between now and when I FIRE. This is easy since the SS tax cap is indexed by inflation so it is just equal to 2013 SS taxes every year I continue working.
I then went to SS web site and extracted what my benefits would be if I start withdrawing when I am 67 or 70.
Then I took 70% of the number to take into account the fact that SS will run out of funding and would be funded at 78% after 2033. I figured that 70% would be a conservative number.
Then what I did is a simple dollar and time weight ROI calculation. Since I am doing everything in 2013 dollars then my Rate of return would be in after inflation terms.
Then I tried various scenarios of when I stopped getting SS benefits which would represent what age my death would be.
This is what I got
Age of With With
Death at 70 at 67
95 2.9% 2.4%
90 2.5% 2.1%
85 2.0% 1.6%
80 1.3% 0.9%
78 0.8% 0.6%
77 0.5% 0.4%
75 0.0% 0.0%
So for me with these assumptions 75 is the break-even age. This means if I plan to live past 75 then I am better of delaying SS until age 70.
Note for me pretty every year I paid SS taxes I paid the max. But I will end up paying SS taxes for 22-23 years whereas SS calculates benefits based on a 40 year average of such a total. So such a calculation would lead to a medium sort of result in terms of benefits.
For my DW, note it is the same thing, she would have worked 17-18 years before she will join me in FIRE. Every one of these years, like me, she maxed out SS tax. But since the average is over 40 year average she will be considered "lower income" than myself so the ROI should be better. And it is.
Age of With With
Death at 70 at 67
95 3.3% 2.8%
90 3.0% 2.5%
85 2.5% 2.1%
80 1.7% 1.4%
78 1.2% 0.9%
77 0.9% 0.7%
75 0.2% 0.2%
I guess same is with her as myself, if she expects to live beyond 75 then she is better of with starting SS withdraw at 70.
When I did my estimates for post-FIRE future, I pretty much assumed our ROI in assets are around 1% greater than inflation. Looks like if my DW and I live beyond 80 then SS would have given us an return better than my assumptions on our other assets.