if I get pd 42Cents/km for business travel, and my employer signs a T2200 form, can I claim up to the 50cents?

CRA states that the limit on the deduction of tax-exempt allowances paid by employers to employees using their personal vehicle for business purposes will increase by 5 cents to 50 cents per kilometre for the first 5,000 kilometres driven and 44 cents for each additional kilometre. For the Northwest Territories, Yukon and Nunavut, the tax-exempt allowance will rise by 5 cents to 54 cents for the first 5,000 kilometres driven and 48 cents for each additional kilometre

No, as I explained in the previous question, the limit to which you refer is the MAXIMUM for which you can be reimbursed without paying taxes on the amount. It does not generate a further deduction when filing. If you would like further reimbursement, you can lobby your company for more.

A little further information: CRA only allows deductions for employee use of a vehicle under the following conditions:

Employees Earning A Salary
If you are an employee earning a salary, you can deduct your motor vehicle expenses if you meet all the following conditions:

* You were normally required to work away from your employer's place of business or in different places.
* Under your contract of employment, you had to pay your own motor vehicle expenses.
* You did not receive a non-taxable allowance for motor vehicle expenses. Generally, an allowance is non-taxable when it is based solely on a reasonable per-kilometre rate.
* You keep with your records a completed copy of Form T2200, Declaration of Conditions of Employment, that has been signed by your employer.

To claim your deduction, you must fill out forms T2200 and T777. However, only include form T777 with your tax return. Form T2200 must be kept in case the Canada Revenue Agency asks to see it.

As you can see, there is no deduction for people who already receive an allowance.

In addition (I didn't copy this bit), even if there were an available deduction, it is only allowed as a detailed calculation of costs (receipts required). They specifically disallow claims for a per kilometer rate.

So sorry, you are out of luck, you'll have to work your employer for more money.