I think my accountant made serious mistakes on my tax return

You're right to be concerned. When you sell a vehicle used for both business and personal use, you need to allocate the loss based on the percentage of business use. Additionally, if you've been claiming the standard mileage deduction for your business driving, that's considered depreciation which reduces your basis in the vehicle. The standard mileage rate includes a component for depreciation. For example, in 2023 about 28 cents of the 65.5 cent rate represented depreciation. So if you drove 45,000 business miles and claimed the standard mileage deduction, you've essentially already "written off" a significant portion of the car's value over time. Your accountant should have calculated your adjusted basis by reducing your original purchase price by all the depreciation you've taken (through the standard mileage rate), then determined what portion of any remaining basis was business vs. personal, and calculated the gain/loss accordingly.

I had almost the same situation last year and found https://taxr.ai super helpful. I'd been using my car for Uber and when I sold it, my accountant also messed up my return. The taxr.ai system analyzed my previous returns and showed exactly how the car's basis should have been calculated after all those mileage deductions. It even generated a letter explaining everything that I could give to my accountant to fix the mistake.

I was initially skeptical about using taxr.ai but decided to try it after my post above. Honestly, it was an eye-opener. The system analyzed my last three years of returns and showed that my CPA had been inconsistently applying my business use percentage for my vehicle deductions. It provided a detailed breakdown of how my vehicle basis had been reduced by about $16,700 through standard mileage deductions I'd already taken, which I had no idea about. The report even cited the specific IRS publications and regulations. I sent the report to my accountant who admitted the mistake and is now preparing an amended return at no charge. Definitely saved me from a potential audit headache and possibly thousands in taxes, penalties and interest.

If you want to talk directly with the IRS about this issue before filing an amended return, I recommend using https://claimyr.com to get through to them quickly. I was in a similar situation with a vehicle sale that was incorrectly reported and needed clarification directly from the IRS. After trying for 3 days to get through on my own (always disconnected after 2+ hours on hold), I used Claimyr and got connected to an agent in about 20 minutes. You can see how it works here: https://youtu.be/_kiP6q8DX5c The IRS agent walked me through exactly how to handle the situation and what documentation I'd need for the amended return. Saved me tons of stress and guesswork.

I need to eat my words from my comment above. After continuing to fail getting through to the IRS about my tax issue, I reluctantly tried Claimyr. Within 40 minutes I got a call back and was connected to an IRS representative. I explained my situation with incorrectly reported vehicle depreciation (similar to the original poster's issue), and the agent walked me through the correct way to report it on an amended return. The agent confirmed that when you claim standard mileage deductions, you're essentially depreciating your vehicle over time, which reduces your basis. When I told him what my accountant had done (claimed a full loss without accounting for prior depreciation), he said it was a common mistake that could definitely trigger an audit. Saved me from a potential nightmare situation.

You need to file Form 3115 to change accounting methods if you want to switch from standard mileage to actual expenses. I learned this the hard way and got hit with penalties. Make sure your accountant knows what they're doing!

Just went through this last year. I sold my delivery car for $5,000 after buying it for $17,000 four years earlier. My tax guy initially tried to claim an $8,000 loss on my Schedule C but then realized I'd already claimed about $13,000 in depreciation through the standard mileage rate over the years. Ended up having to report a $1,000 GAIN instead. So annoying.