Awd when?

Read the SUV hybrids will use it on launch from a stop. That may only last to 5mph or so under normal conditions. They may have a mode where it can be engaged at low speeds for 'off road' use.

Otherwise, it only comes on when the front wheels start slipping.

 

I've noticed on the drivetrain display that it activates in relatively slow turns onto an intersection from a stop for just one side.

 

Turning it on for low speed may not be an option. Only heard of that with Toyota SUVs, and not all of them. If needed for snow or dirt roads, it should just come on whenever it is needed.

The EPA tests are very laid back; acceleration rates well over 10 secs to 60mph. The rear motor adds weight, and unless it is an induction motor, it applies a braking force when it isn't powered. Even with an induction motor, there is likely more friction in the rear axle compared to the FWD model. That's why the rating is lower.

 

The AWD is computer generated, meaning it's always on but engages wheels differently as it senses tire slippage.
I use the B gear also in snow.

 

If you look at the shifter, you’ll see two down arrows. To the right is the B gear to the left is the D gear.

 

The two core EPA test cycles where developed from drive patterns in the 1950s and '60s. The highway one is not at a steady speed, and has numerous slow downs and re-accelerations.
https://fueleconomy.gov/feg/fe_test_schedules.shtml

That said, what I called magnetic braking could be the main culprit. Proper name is https://en.wikipedia.org/wiki/Eddy_current_brake. Many casting fishing reels make use of the principle to control line out speed.

Most AWD EVs do not power both motors at the same time; inefficient to do so. When unpowered, the permanent magnets in the secondary motor apply a braking force to that axle. It's like having the friction brakes drag. More powerful the motor, the more braking, which is why the new AWD Prius sees a bigger penalty than the old one.

This can be avoided by using an induction motor which doesn't have permanent magnets(Teslas), or a clutch on the motor(upcoming Ram PHEV). The first is less efficient when powered, and the later adds complexity to the system, but overall can be worth doing. Haven't seen anything to suggest Toyota uses either.

Something to consider in any car if best efficiency is the goal. The improvement in a Prius might make it hard to justify the cost though.

 

My 16's are at least half the weight of the OEM

 

It's the same 3mpg penalty to combined for each. The gen5 is hurt more in the city rating. The gen4 AWD is 170 pounds heavier. There is likely the difference in it having NiMH instead of Li-ion in effect.

I must have been thinking of Toyota hybrid SUVs. When FWD models were available, the mpg difference was smaller.

 

Snow time is the one time I'd be inclined to use ECO mode (if it still exists on gen 5), for it's more sluggish response. And leave the "gear" in D.

 

Good answers! This will be my 3rd winter in my 21 AWD Prius and I'm much more confident to drive it in the snow and ice. The computer traction system controls when the rear wheels' motor engage automatically...I notice mine only engaging when driving up snow or icy hills and the front are slipping.
I asked my mechanic that, since it's an AWD, do I still need to rotate the tires every 5,000 miles and he said yes because the front tires, still, take most of the wear and tear even on an AWD car.
For deeper snow, however, I will take the 4x4 Tundra over the Prius.....a lot of snow can get underneath and high end the car making you get stuck...even with AWD. We have many here in Colorado with AWD Subaru's who learn this lesson (I have a tow rope in my Tundra to pull people out when they get stuck....lots of Subaru's and Honda's have been helped!)

 

.

 

That's not going to do what you think.