That's an excellent point your bring up in your comment. I was previously unaware of this particular website. That said, we do need to be careful to distinguish between the General Services Administration and the Internal Revenue Service. Just because they are both government agencies, this doesn't necessarily mean that one agency has a say over the enforcement rules or practices of the other.
In particular, although the GSA does indeed have a great deal of specific and helpful information to offer, please keep in mind that the GSA webpage we are viewing is one that lists "Privately Owned Vehicle Mileage Reimbursement Rates."
That is, the GSA figure is the dollar amount, per mile, that the private owner of an aircraft might legally expect the government to pay (i.e., reimburse) them for the aircraft's use in travel for purposes of official government business. This is not quite the same thing as the IRS providing us, as taxpayers, with an average cost of operating a ground vehicle per mile, for purposes of income tax expensing.
The GSA figure might approximate your actual operating costs per mile, and then it again it might not (I suppose that would depend on your particular aircraft). The proper tax approach, however, with aircraft, is to use actual operating costs, if you have those records and can distill that down to a final annual figure. In lieu of complete recordkeeping, the IRS does have provisions for what to do in case you don't have records for a particular tax item. Invoking them could allow you to use the GSA number as an expense approximation; in lieu of anything more accurate. But, if you can calculate actual figures, I would respectfully advise you to use those (provable) costs as being the more correct method, from a tax-accounting perspective.
