I think all cars and bikes have the needle showing less than the full mark on the fuel meter. My GP MJD and my Honda unicorn too shows the needle less than the full mark. It is not an issue actually (when I took my car last time from Prerana after the issues resolved, I filled in 42.02 litters of diesel, that means the car used the diesel to the untouchable 3 litters? Nay. It is actually came up to the fuel cap). Ask the SC to replace the tank under warranty and tell them the damage is caused in the SC not by you as if you noticed it immediately from service.