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.

A stock '66 Mustang hardtop had a curb weight below 3000lb, in the lightest configuration close to 2500lb.

Less mass to move will do a lot for efficiency just like aerodynamics will.

Of course, you will also die or be horrifically maimed in an accident in a 1966 Mustang that you might walk away without any serious injuries from in a modern vehicle.