Personal Vehicle Used (Almost) 100% For Business

I have a personal vehicle that I use almost exclusively for business. If I put 10,000 miles on it in a year, 9,500 are for business. I track my mileage religiously, but accountant always tells me I can't report all of that business mileage because it will trigger an IRS red flag. He usually reports the business mileage somewhere below 50%. I feel like I'm getting cheated out of a larger deduction. My business miles are legitimate! Does anyone else have this problem or do I need a new accountant?