I drive a 2003 Mazda that costs me about $0.15/mile to drive and get reimbursed at $0.555/mile give or take. That cost includes buying the car, maintenance, repairs, taxes, tags, etc.
Driving a used car, you can make a bit of money on the mileage reimbursement.
While 0.45/mile is lower than the IRS figure, you'll still make about $20/hour driving on the interstate.
Let's see, 250 miles * $0.30 if your operating costs are similar. So you are making $75 one way, or about $20-$25/hour depending on how fast you drive after you pay your costs.
If you drive the company car, you make $0/hour on the mileage.
You don't pay taxes if you expense your mileage. You may pay taxes if you have a car allowance. Previously, I got a $450/month car allowance, but could then deduct my work mileage, as the allowance was taxable. We went to claiming expenses on our work related mileage. I no longer got the allowance, and no longer had to document my mileage for the IRS.
I still keep a log, but that is now for my expense report.
I think it comes down to what you value most. If it were me, I'd milk the Avalon for a much company money as possible, putting the $0.30/mile you are netting into a saving for a replacement car, or repairing the A/C and any other bothersome issues.