The car most likely is fitted with winter tires. Owners often opt to not purchase tires rated for high speeds (should be Y index for the summer tires but they went for V (thus 240km/h) for the winter tires).
In a nutshell your tires are only rates for speeds up to 240km/h. I doubt that Avis would check the various data sources to verify if you exceeded the speed limit but it's a matter of save driving.
I've exceeded 240km/h on my winter tires but with higher wear on dry roads and safety concern I just stay under.