Can I refuse to pay the notice in lieu computed based on gross salary and working days?

I have questions about my resignation with short notice.  The notice period is 3 months but I can only give 1 month.  Therefore, I have to pay them in lieu of notice for 2 months.  However, I am surprised that they also request me to include the car allowance in the payment.  In the employment contract, it is no mentioning about the monthly car allowance is part of the salary.  Can I refuse to pay the car allowance part?
HR told me that they calculate by number of work days but not calendar date.  In that case, they charge me one more day of salary (including my car allowance).    Can I refuse to pay this one day payment?

The notice in lieu is computed based on the gross salary rate. Your car allowance may classified as one of your gross salary components.
No, because your HR is correct - notice in lieu should be based on number of working days.