Hello, I guess that's a combination of air resistance, engine power and gearbox settings. Have you already tried going for some AI race at the Daytona Oval, setting the rear wing to the lowest value possible and using some slipstream? I feel like especially older GT3s should be able to do more than 270, at least in that situation, but I'm not 100% sure.
A bit more context would be helpful (car, track, setup etc). In general the older cars tend to have more top speed than the newer ones but in the right circumstances (e.g. bathurst (perhaps with some setup tweaks aimed towards more top speed)) even the new ones should generally be able to get a bit higher than 270. 280 might be a different story but i also dont know if there are any places where gt3 cars reach those speeds without slipstream in real life
alors pour le contexte J'ai essayé plusieurs GT3 R8 GT3 evo II, AMG GT3 evo, Porsche GT3-R 992 sur différents circuits Paul Ricard 1A, nurburgring nordschleife a chaque fois elles plafonnent au alentours de 269 km/h alors que la cartographie moteur indique en fond de 6ème 295 km/h
The top speed in the setup screen is just a theoretical value for the gearbox ratios. It will always be lower than that out on track for various reasons.
As majuh says, those 295kph wont be reached. As for nordschleife and paul ricard, both of those long straight are mainly uphill which doesnt help. Try bathurst or maybe daytona to get a better idea of what the cars are capable of on default setup