Robert M. Gary wrote:
"some companies.. including the federal gov't, already have in a
"mileage
allowance" "

Actually, this is something set by the IRS. If a company pays you more
than the mileage allowance (this is for cars) you must show receipts to
the IRS that your vehicle costs more to operate, otherwise you will be
taxes on the difference as ordinary income.

About two years ago I recall reading that there was a rate of
$0.975/mile for use of GA aircraft. Auto rate was about $0.37 at the
time. I'm not sure if this was official IRS rate or a contract rate set
by GSA or somehting similar.

The IRS would actually be quite irritated if you took a car milage
against an aircraft unless you also provided documentation that you
spent at least that on the aircraft. You can not just say "give me the
mileage allowance and don't ask how I got there".

If this is just an occasional usage I'd just claim it as car mileage
and no one will be the wiser. In the odd event you get audited, the
actual rate will surely be significantly higher.

-cwk.