Actually, the question should be..."How FAR can it travel on the batteries after the gas tank is depleted? Every time you slow down, it makes more juice!
So it only can re-use the energy of slowing down?
How does that help HIGHWAY mileage?
Regenerative braking has been used in the railroad industry since the early 20th century. It is commonly referred to as “Dynamic braking”.
Locomotives commonly referred to as “diesels” are actually “diesel-electrics” with the diesel engine driving an generator or alternator which furnishes electricity for the traction motors which are geared to the drive axles.
By reversing the polarity of the field winding of the motors, they operate as “mini-generators” producing electricity which is wasted as heat through air cooled resistor grids on the roof of the locomotive. The mechanical resistance of the motors is used to hold train speed on a downgrade, or slow the train on more level sections of the railroad and reduce the use of the air brakes.
On some straight electric locomotives the regenerated electricity was returned to the overhead catenary system. The Chicago, Milwaukee, St. Paul & Pacific RR used this system on their two electrified divisions in the Rocky and Cascade mountain ranges.
GE has a prototype diesel-electric locomotive which uses battery banks to store the electricity so that it can be used on the next acceleration after braking. The biggest obstacle to wide scale application of this technology is the weight of the battery banks which makes the locomotive exceptionally heavy.
I got nearly 34 mpg highway on one tank of gas on my 2000 Acura TL by carefully watching my driving habits.
Instead of jackrabbiting up to tailgate the next guy ahead of me on the freeway like most other people seem to enjoy doing, I maintained plenty of following distance, left home earlier, planned ahead for ramps and lane changes, and didn't touch the brakes at all for about 90% of my 50-mile, 65mph commute.
I can hardly imagine what kind of MPG I would have gotten if I could have driven 55mph without causing a 50-car pile-up.