Found this while fooling around doing donuts in the pits.

You change tyres and repair your car in 0 seconds in the fashion shown in my demonstration replay. No refuel is done though. I had starting fuel at 50%, 60% at refuel but no addition can be seen in the replay.

Not all pitlanes are suitable for this action though, there are no walls close enough in tracks other than Aston (both pitlanes) and South City. Blackwood looks like it'd work but I wasn't skillful enough.