If the national average for mileage reimbursement is 50.5 cents/mile, how much of this is for mileage and how much for wear/tear on vehicle?

There are 2 methods to figuring deductible car expenses: the actual expense method or the standard millage rate method. You should choose whichever method is the greater expense. If you do use the standard mileage rate you can also include parking tickets and tolls that were incurred while using your car for business purposes.

Here is more information from the IRS site so you can see which works for your specific situation:
http://www.irs.gov/taxtopics/tc510.html

What I want to know is if the mileage reimbursement is 50.5 cents/mile, how much of that is for gas and how much is for wear and tear on a vehicle? For example, is .30 cents for gas and 20.5 cents for wear and tear? We are a small company and we have company credit cards. We also have employees that use their card for gas and then fill out an expense report for mileage and expect to get mileage x 50.5 in full. Their do not deduct the gas that they purchased on the company credit card. When told that all of the 50.5 cents is not for gas they want to know how much is this figure is gas and how much is wear and tear. So, I need help with splitting the amount into these two categories.

I don't know if you can split them up into two separate categories because everything I've seen groups them into the same bucket.

I don't think that they break it apart, it is a lump sum grouped together.