wrote:

I fly a 172 F.

The fuel selector says to select single tank operation at or above 5000
feet.

172's occasionally get a form of vapor lock above 5000.



If you leave the selector in the "both" position, under 5000 for any
prolonged period of time (couple of hours) the right tank appears to go
down more quickly than the left. This is confirmed at fillup.

On my 182 it was always the left because the vent was on that side.



Here's my question, if the right tank runs out, but there is still fuel
in hte left tank, selector is on "both" what will happen.

Nothing.



As part of my pre-flight, I always check operation on each tank, then
switch to "both" before my run up.

I never moved the selector from both except when I needed to fill the
tanks and make sure I got all the gas in there that the tanks could
hold. To do that you had to move the selector to any position but both.
I once started the plane in front of the hangar and then turned the
fuel off to see how long the engine would run. It ran about 30 seconds.
After that I never turned the fuel off. I certainly never checked the
fuel selector at runup. You have no sure way to know when you are
drawing off the other tank after selecting it. Plus with full tanks the
gas will transfer from one tank to the other thru the vent line across
the top of the fuselage if the plane is slightly out of level.