Fuel Effect
VerifiedA car starts the race heaviest, with a full fuel load, and gets quicker lap on lap as that fuel burns off.
A Formula 1 car is heaviest at the start of a stint, when it's carrying all the fuel it needs. Every lap burns some of that off, and the car gets lighter and faster as a result, worth roughly a few hundredths of a second per lap for every kilogram shed.
That means a driver's very first laps on a full tank are never their true pace. Someone running long on old fuel can look artificially slow next to a car that's already burned off weight, even if the underlying pace is similar.
To compare drivers or stints fairly, that fuel weight has to be corrected out of the lap times first. It's one of the basic cleaning steps behind any honest read of race pace.