If you are audited, the IRS does not have to give you any deduction you can't prove. The written instructions in publication 463 require a diary or log of some kind that includes the date, mileage, and business purpose of the trip, that is contemporaneous—kept at or near the time of the business trip. The farther away from the trip, the more the IRS is likely to question the accuracy of your recollections. When you sit with an auditor in 2021, going over your 2018 tax return, how will you convince them that your estimate is fair and accurate?