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I would need to make some assumptions... let me know if any should be corrected.
you both file as single and neither has dependents.
you both use standard deduction
neither of you has any additional credits or deductions.
your finance's adjusted gross income $80,000 - $8000(401k contribution) = $72,000, and gross tax obligations ~$12,100
your's adjusted gross income $70,000, and gross tax obligations ~$11,600
Total tax obligations ~$23,700
If you file joint tax return - result will be the same.
However, if any of you itemize, claim student loan interest, deduction, educational expenses, have dependents, etc - result might be different... ... let me know if any should be corrected.
Married couple do have some tax saving if they have different taxable income or one spouse doesn't have income at all - in your situation income amounts are very close and even if there will be some saving - that would be minimal.
From my practice - most married couples pay more taxes compare if they would not be married.
- if you have educational expenses and claim a credit - you will not be able to use the credit filing joint return because Adjusted Gross income will be higher.
- if you claim job related expenses - there is a floor limit of 2% AGI - in your situation $70,000*2%=$1400 - means only expenses above $1400 are deductible; filing jointly - the limit will be $142000%2%=$2840 - means you would loose $1440 deductions.
As I mentioned above - if you are using any specific tax credits or deductions - they might be affected.
You might know about "Marriage Tax Penalty" - means almost all married couple owe more taxes than if they were not married. In 2003 the Jobs and Growth Tax Relief Reconciliation Act of 2003 eliminated difference in standard deductions and corrected tax brackets. However that provision set to be reinstated in 2010, unless Congress makes tax relief permanent.