Not implausible he hit those numbers, electric car ranges depend on a lot of factors, and he likely had a very small and favorable sample to get him to 258 wh/mile.

Some factors to consider: 1. Winter gets 20-25% less range 2. Poorly inflated tires get 3% less range 3. Driving at 60 mph requires 80% more energy per mile than driving at 20 mph

So if you take it together, test drives around town with proper tires in a Tesla 3 in summer, can get 130% better range than a tesla in winter on a highway with poorly inflated tires.

Most Tesla's average range is something between those two, say 260 wh/mi. But they can get below 200 in good conditions (summer driving at 30km/h). So if you take those good conditions and put a non-aerodynamic mustang body on it, it can do the same as a Tesla in average conditions.