Here are your estimates for 2009
1. Calculate your estimated gross income
Your income - $80000
Your wife's income - $41730
Your total expected annual income in 2009 - $121730
2. Calculate your taxable income
$121730(adjusted gross income) - $23000(itemized deductions from above plus additional state tax as your wages will be more) - $10800(personal exemption for three persons) = $88,000
3. Calculate your tax liability (based on 2008 table with inflation adjustments)
Your estimated federal income tax $14600
You will likely claim child tax credit $1000
That will reduce your estimated tax liability to $13,600
4. Calculate total withholding (also based on 2008 tables)
Yours - $416 * 26 pay periods = $10816 (assuming you will file W4 as married with zero allowances)
Your wife's - $114.60 * 26 pay periods = $2980
Your total estimated withholding is $13796 (actual 2009 withholding will be a little less)
So far your withholding will be about the same as your tax liability.
Now - I have some concern about changing your company's reimbursement policy to a monthly reimbursement to cover the cost of the car payment, insurance, gas, and maintenance.
You need to determine if that will be an accountable reimbursement or not accountable.
Accountable generally means - you are required to report actual expenses and return all leftover.
Not accountable - means - you do not need to report your spending, but in this situation all reimbursement should be considered as your taxable income. You still will be able to deduct your expenses on schedule A, but 2% floor limit will be applied.