Why do F1 cars lose pace in a race compared to qualifiers?

The main reason is that in qualifying they run with the minimum quantity of fuel required to do 2 or 3 laps whereas at the start of the race they have a little over 100kg on board which they need to complete the race.

This 100kg of fuel is a substantial fraction of the car’s total mass. The rule of thumb is that every 10kg of fuel costs about 0.3s of lap time so cars can be something like 3 seconds a lap slower just because of the mass of fuel.

In qualifying the driver and team will try to extract the absolute maximum performance for the car for one fast lap, this puts a lot of load on the engine and tyres which generally can’t be sustained for an entire 2 hour race. Equally driver will try to be on the absolute limit for qualifying laps, this carries an increased risk of errors and that risk/reward calculation can change for a 50 lap race compared to one lap.

Tyres are also a factor, there are several different dry weather tyre compounds available per race weekend each with a different balance of performance vs durability and teams are required to use 2 different compounds during the race ie make at last one pit-stop. They are also required to start on the tyres they qualified on (usually the softest available for the front runners) so they will often end up on a harder compound later in the race when the cars are lighter.

The current F1 tyres usually give best performance when nearly brand new (although they may be given one or two fairly gentle laps the day before as a heat cycle can make them work better). and as the top 10 cars are obliged to start on the tyres they qualified in the first set will already be past their best by the time they start the race and with the current tyre restrictions qualifying tends to mean that all of the softest tyres have been used for at least one hard lap before the race starts.

Teams a make a calculation on tyre strategy, changing tyres costs time (usually about 20 seconds overall) and so sometimes it is better to drive a bit more conservatively and pit at a more advantageous time to improve the overall race time. typically t leaders will not want to pit untill they have a decent gap to the midfield to avoid getting stuck behind much slower traffic.

Also during a race driver need to deal with other cars, if they are behind a slower car they may be held up, the lines used for attacking and defending tend to be slower than the optimum on a clear track plus the turbulence produced by cars in front tends to disturb aerodynamics and cooling adversely affecting performance.

There is also the fact that by the last few laps of a race when fuel loads are close to qualifying levels the tyres are probably past their best and it is unlikely that there is much point in drivers pushing to the absolute limit (in terms of both engine performance and drive risk-taking) as with a few laps to go they will either be comfortably ahead of any competition and trying to avoid any silly mistakes or in a close fight which tends to slow both cars.

Equally under modern regulations engines have to last about half a dozen race weekends or more and using the fastest engine modes takes life out of them so they won’t use the most demanding modes unless there is a clear advantage to be had in doing so.

In some circumstances cars can set race lap times which get close to qualifying eg

• they change to a new and unused set of the softest tyres very near the end of the race, this is unusual but can happen if they get a puncture or other technical problem obliging them to pit or if they are on an unconventional tyre strategy .
• they have a set of unused softest tyres available near the end of the race .
• track conditions significantly improve from the Saturday to the Sunday, obviously if qualifying is wet and the race is dry this is pretty much inevitable.
• they have a good reason to push hard in clean air at the end of the race eg if the car in front has a time penalty and they only need to get within 5 or 10 seconds to jump a place without having to overtake.