Reduction in benefits with reduced wages

Having higher average earnings will give you higher Social Security retirement benefits, but the benefit increase is not proportional to your higher wages. In your case, if you already have 25 years of relatively high wages, followed by 13 years of lower wages, your retirement benefits may not go down that much.

Social Security uses the highest 35 years of indexed earnings in a benefit computation, along with two "bend points" that reduce the effectiveness of higher wages on retirement benefits.

Probably a bigger factor affecting your retirement benefits which is under your control is your ability to delay receipt of retirement benefits.

For example, see this scenario :
http://www.ssa.gov/OACT/ProgData/retirebenefit1.html

It shows a scenario where one worker A has high early earnings, low late earnings, but retires early. Worker B has low early wages, high late wages, but retires later. Worker A winds up ahead in monthly benefits with $1,510 vs. $1,605 for worker B.