I receive a fixed auto reimbursement and variable mileage reimbursement from my employer. If I have read right I can deduct the difference in the total reimbursement.

Yes, enter the total miles and then TurboTax will ask for your employer's reimbursed amount.

If you are an employee, vehicle (Job-related) expenses are reported on Form 2106 (Employee Business Expenses).

• Sign into your account and open your return by clicking the orange Take me to my return button.
• Type vehicle expenses, employee in the search box and click search..
• Click the Jump to vehicle expenses, employee link in the search results.
• At the Tell us about the occupation you have expenses for screen, enter your occupation.
• Click Continue, and follow the onscreen instructions.
• If you land on the Job-Related Expenses Summary screen instead, you can either Edit expenses for an existing job or click Add Another Occupation to enter expenses for a new one.
• Continue to the enter mileage screens and the rest of the  screens until you get to the Reimbursements for your expenses screen, answer Yes. Continue.

Tip: If the only work-related use of your car is commuting back and forth between your home and main workplace, do not enter any vehicle mileage or expenses. The IRS doesn't let employees deduct commuting costs.

See the screenshot below.