Would highway range be increased by an overdrive gear?

Would highway range be increased by an overdrive gear?

ICE vehicles can get much better HWY economy than City, even at 80mph because of transmissions with 7 or 8 gears now that keep rpms down. I wonder if the range on the MS wouldn't take such a dive at fast speeds if the motor could turn much slower at highway speeds with an overdrive gear? I realize it would make the transmission more complicated, but I wonder if a 2-speed transmission would change anything. Obviously wind resistance increases exponentially with speed, but with so much torque at low rpm, would the motor use less electrons at half the rpm? Any electrical engineers care to comment?

DickB | May 17, 2013

Short answer is no.
As you increase the final gear ratio, the motor would need more watts to move the vehicle.

txjak | May 17, 2013

No, not for range. I would imagine that the motor doesn't lose much torque with rpm, but there is an upper limit to the number of revs it can stand.

About the only reason I can think of for an overdrive gear would be for places like the Autobahn in Germany, where you might want to cruise at a higher speed than the motor will currently will.

Jewsh | May 17, 2013

Initially Tesla tried to use a gearbox but due to the torquey nature of electric motors these transmissions failed prematurely. I would expect any further attempts to provide something other than direct gearing to display similar results, unfortunately.

jat | May 19, 2013

Drag is the primary issue at highway speeds, so it won't help that. Resistive losses in the motor windings are related to current and wouldn't change by running at a lower RPM but the same overall load.

The only gain would be if the motor were running in a particularly inefficient RPM range, and while I haven't seen charts for Tesla's motor, typically AC induction motors have a pretty flat efficiency curve until near their top RPM. So, unless you are driving over 100mph, it wouldn't likely make much difference.

nickjhowe | May 19, 2013

+1 Jat.

A higher gear would give you more torque at higher speeds, as the torque curve is linear to 6000 RPM and then begins to fall off; if you could get the motor back below 6000 at 70mph you'd be better off, but only if you are on the track. There's really no need for that in everyday driving situations.