In article et,
Mike Spera wrote:

If you claim the entire trip's mileage, are reimbursed, and use the
reimbursement for fuel, have you just conducted a "commercial" flight
under your aircraft insurance carrier's eyes? As I recall, fuel expenses
must be shared by the occupants to avoid being a commercial flight under
FAA rules.

The FAR is this:

61.113 Private pilot privileges and limitations: Pilot in command.
[...]
(b) A private pilot may, for compensation or hire, act as pilot in command
of an aircraft in connection with any business or employment if:
(1) The flight is only incidental to that business or employment; and
(2) The aircraft does not carry passengers or property for compensation or
hire.

Let's look at that carefully.

"A private pilot may, for compensation": You can be compensated for your
expenses (i.e. claim mileage, per-diem, or even your FBO's full hourly
rental cost).

"... or hire": You can even continue to draw your regular salary or wage.
I get paid an annual salary. As far as the FAA is concerned, if I fly
myself to Chicago, I'm perfectly OK marking down my time for that day as a
normal work day.

"The flight is only incidental to that business or employment": This is
where most people start to get confused. Incidental means the flying is
not an essential or required part of your job. Your boss says, "Be in
Chicago on Tuesday to attend a meeting with our client". The reason for
going to Chicago is because your boss needs you to meet with a client. You
could have driven, bought a ticket on United, rode your bicycle, or stuck a
bunch of stamps on your forehead and climbed into mailbox. The mode of
travel wasn't the important thing; the getting there was.

"The aircraft does not carry passengers or property for compensation or
hire": This is another tricky one for many people. Note that it doesn't
say you can't carry passengers or property. It just says you can't do
those things for compensation or hire. So, when I fly myself to Chicago,
if a co-worker, who also has to meet with the same client, comes with me,
that's fine.

One the other hand, if I say to my co-worker, "Hey, Joe, I'm happy to let
you come with me, but the 47 cents/mile rate I'm getting on my travel
expense report only covers half my real flying expenses. If both of us go,
we can both claim 47 cents per mile and if you give me yours, I'll just
about break even", now I'm in trouble. I'm carrying a passenger for
compensation.