Sure, there’s always going to be outliers. Most people live and work in the same metropolitan area though - they’re not driving 50,000km+ a year. Besides, having a vehicle with 5 times the effective lifetime is going to be a big win regardless of how much you drive it.
The signing ensures the integrity of the data, whether using a public block chain or not.
The signed document can be distributed as widely as you’d like - it doesn’t need to be attached to a block chain to do this.