If you are suggesting that the mileage method, even at using only 75% of total miles as a deduction, means that the taxpayer does not need to maintain a mileage log supporting the miles being deducted, that is not correct.

Any kind of mileage-based calculation has to be supported by a log.

Note that a person is drawing a comparison between the total miles incurred, the total miles used for business, and the 75% portion of those.  Without a log, or base to establish a comparison, there can be no determination.

The use of the cost-based determination is merely a system of supporting the paid bills, like normal, and then take 75% as business...…...or exclude 25%...….same thing.

The question for which I have never received an adequate answer to is...……..without maintaining a log indicating miles, ie establishing a base to determine "use"...……..how can one say with authority that the vehicle is actually used more than 50% for farm?  AND, even if more than 50% use can be established, how can one say with authority that the actual farm use is more than 75%, so that using 75% is appropriate?

And, since I have seen no court cases on it, and since Grassley and the Boys did not ask me for my opinion when they ran thru the Revenue Procedure years ago...…….I will probably never receive such answer.