I think you are off-base from the perspective of the intent of the procedure.

If a taxpayer is going to use the "mileage method", that method is predicated on the use of a list of miles.  the mileage method in section 274 indicates a deduction based on miles x allowed rates.  This applies to any taxpayer, not just a farmer.

Therefore, there has to be a presentation of miles to use the mileage method.  Number of miles x allowable rate = deduction.

Resultantly, a mileage log has to be used with the mileage method...….the number of miles has to be established.

With respect to the "usual method"...…...ie actual costs...……. a taxpayer still has to present a determination of the percentage of business use.  IRS/Supreme Court has settled on a "contemporaneously prepared record" indicating business use...…...translated by IRS as being a mileage log.

Based on the miles in that log, the business use/% is established, and that % is applied to the actual costs related to that vehicle in order to determine amount of deduction.

So...…...with either scenario...…..actual costs or mileage ........the deduction is predicated on a listing of miles for business.

The 75% rule came into being when the farmers bitched about having to keep a mileage log...….for any purpose.

To accommodate the farmers, and only farmers, Grassley and the IRS came up with the 75% rule...….if the vehicle is used daily, etc etc etc then one can apply 75% against the actual costs and use that as the deductible amount...…….and no log will be required.

However...…..since there is no mileage log to establish business use, then there is no established number of miles, and half the formula is missing...……….. miles of ???  x allowed reimbursement rate = ???.  Mileage method is not useable.

With the actual cost method...……. miles are not needed...….. 75% by procedure x actual costs as presented in books = amount of deduction.

Edit:

I mis-spoke.  The "procedure" noted above is NOT a Revenue Procedure...…..rather, it is of higher authority...…..namely a Regulation.  It is 1.274-6T.