Formula 1 practice at non-sprint weekends is a well-known format - teams have an hour in both FP1 and FP2 on Fridays and then a third 60-minute session on Saturday before qualifying.
But second practice at the Mexico City Grand Prix will take on a different format as Pirelli conducts a vital test ahead of its 2025 tyre compound homologation.
The change mirrors the one made for last year's event, where the Italian manufacturer trialled a prototype C4 compound ahead of the current campaign.
Pirelli needs real working data when finalising its homologation ahead of the new season, which will again see F1 travel to 24 destinations across the globe, and with a lack of in-season testing time, this weekend's practice session has been earmarked as a chance to gather information.
As explained by Pirelli, the session will be "entirely given over to validating" the tyre range's softest compounds for next year: the C4, C5 and C6 compounds.
To facilitate the test, FP2 will be extended to 90 minutes, with all drivers and teams obliged to follow a run plan established by Pirelli's engineers.
So, in addition to the usual dry tyre allocation for the grand prix weekend, drivers will be given two more sets of tyres. One will be a test control tyre - i.e. the same compound as the soft in the normal allocation - and the other will be a 2025 prototype, which Pirelli has confirmed was homologated in September.
The specified programmes will include a qualifying simulation run and a race simulation run, with "every team running the same number of laps with the same quantity of fuel on board" - dependent on which run type is being performed.
These tyre sets will be identifiable as they will not run with coloured sidewalls.
A complication for the test is that some contracted race drivers will be making way for rookies to complete one of the regulation-mandated rookie runs in FP1, with a lack of on-track time compared to rivals potentially putting those that stepped aside at a disadvantage.
Pirelli has counteracted this by handing those affected an additional set of medium compound tyres for FP2 and allow them 30 minutes of 'free' running, thus only obliged to complete 60 minutes of tyre testing.
Data from the test will be analysed before the post-season group tyre test, which will take place at the Yas Marina Circuit in Abu Dhabi.
With what is essentially only two hours of running to gather race-representative data, teams will face a similar engineering challenge to that of a sprint weekend, albeit with slight respite from the extra practice session before parc ferme is locked in.
Why has Pirelli chosen Mexico?
While you would expect the extra time to be a profit for teams and drivers for the weekend, it actually doesn't work out that way for everyone given the requirements of the test.
As Haas driver Nico Hulkenberg's race engineer Gary Gannon told Autosport, drivers must not get suckered into changing the balance of the car off the back of running on the prototype tyres.
Gannon also pointed out there was no advantage from running two contracted racers in FP1 over those testing rookies, explaining: "If you are not running a young driver in FP1, you are not able to utilise the extra time, so for us, running two racing drivers in FP1, in FP2 we are only doing the Pirelli tyre test programme.
"So FP2 is lost in terms of our understanding and use for the race weekend, so we have to do all of our Friday learning in FP1."
The Autodromo Hermanos Rodriguez is an outlier on the F1 calendar due to its track layout and high altitude, which would suggest it as a strange option for a tyre test.
But this is exactly why it has been chosen and Gannon added: "Pirelli's tyres need to work in all conditions, so it is particularly important to find out if they are particularly weak... they may have a compound that works great at Barcelona where there is a lot of tyre energy and the tyres work because you are always pushing them.
"Here is one of the extreme cases and so we have a good baseline, we have the baseline tyre and we have the morning running so they will know what the grip should feel like. Also, the track improves a lot in FP1, that's why you do the test in FP2, because if you try to do the test out of the box in FP1, the track is changing very dramatically as it gets cleaned up and rubbered in.
"In FP2, the conditions are very stable, so you can find out if this new compound or new construction they have come up with has some weakness on very low grip or smooth tarmac surfaces.
"So they need to be trying all these outlier circuits because that's where you will have a miserable weekend if you deliver tyres that only work in low-downforce, for example."
