Are you claiming Vehicle Expenses for a Business? If so, you can claim Actual Expenses this year and claim the new transmission as a Vehicle Expense.
If you use the vehicle 100% for Business for instance, you can claim 100% of the expense; or allocate according to your Business Use %.
However, you could also set up a separate asset for the transmission. Either way is acceptable, depending on what works best for you. You can select five-year property, same as a Vehicle.
Here's more info on Business Assets.
Here's more info on Business Vehicle Expenses.