On a technical note, you mean range, not mileage. I don't think that anyone is claiming that the Model 3 will have electrical motors that are twice as efficient as the Leaf's motors or anything -- the claim is that it will have batteries that hold more energy than the Leaf's.
I think you're right to be at least a little skeptical of the Model 3. For one, it seems like something of a stretch to say that they'll be on-the-market in 3.5 years with a vehicle that will feature batteries from a factory that hasn't even found a site yet.
That said, I imagine that the Leaf will have a better range in 2017 as well. It seems clear at this point that better battery tech is the final hurdle for electric vehicles. It doesn't seem completely implausible that with a lot of resources thrown at the problem, they'll be able to lower the cost of increasing the energy capacity of batteries in the next few years.
I think you're right to be at least a little skeptical of the Model 3. For one, it seems like something of a stretch to say that they'll be on-the-market in 3.5 years with a vehicle that will feature batteries from a factory that hasn't even found a site yet.
That said, I imagine that the Leaf will have a better range in 2017 as well. It seems clear at this point that better battery tech is the final hurdle for electric vehicles. It doesn't seem completely implausible that with a lot of resources thrown at the problem, they'll be able to lower the cost of increasing the energy capacity of batteries in the next few years.