Is there a way to fix a ridiculously high mileage deduction?

Hi, Robin. QuickBooks uses the standard mileage method by default to estimate your tax deductions. It is calculated by multiplying your total business miles by the current rate per mile set by the IRS. You'll want to refer to their website for the latest mileage rates to verify your deductions.  

On the other hand, if you're using the actual expenses method to manually calculate the amount you actually spent, I recommend consulting a tax expert. They can help you identify exactly which transactions caused that discrepancy.

Please don't hesitate to post again in the Community forum if you have other QuickBooks-related concerns.