Petrol-fuelled power still comes from a 1.8-litre engine running on the high-economy Atkinson cycle, but it now features a two-level cooling system, which restricts flow during the now-quicker warm-up, when the coolant is further heated by exhaust gas.
There's an electric water pump, too, and other refinements that together give the engine a thermal efficiency of 40%, claimed to be the highest ever for a production engine. The hybrid drive's CVT transmission is smaller and lighter, and the two electric motors working with it have a greater speed range, which makes the Prius up to 14% more efficient at high speeds.
Other numbers include 20% less power loss in the transmission, an electrical inverter that's 30% smaller, and a new nickel-metal hydride battery pack that's 10% smaller - it now lives under the back seat, allowing full hatchback versatility - and the ability to take 28% more charge per unit of time.
What's it like?
Visually it's like a smaller, calmer version of the Mirai fuel-cell car. Beautiful it is not, but it's certainly striking with its steeply-rising waistline, blacked-in rear pillars and spiky light designs. Slippery, too, with a Cd of just 0.24. An automatic shutter for the lower front air intake helps here.
Inside, that lower hip point doesn't compromise the view forward because the scuttle is lowered even more. Ahead of you is a smooth facia with the usual Prius central instrument display below the windscreen, while the centre stack fronts the padded crash roll and houses a large TFT multimedia display.
Below sits the transmission selector in imitation transparent-blue carbonfibre, and trimmings in piano black and satin white give an air of calm. Most surfaces that you'd hope to be padded are, except in the base model. This feels quite an upmarket car now.
There's plenty of headroom under that rounded roof, plus ample rear leg room and a decent boot. So far, so worthy, but the surprise comes when you drive it. Toyota's promise of an involving, entertaining drive rings as true here as it didn't with the deeply disappointing Lexus CT200h.
The 121bhp powertrain responds vigorously from low speeds, helped by the greater slug of electric motor assistance; it will even spin its wheels in Power mode. Eco mode equates to the old one's Normal, while the new Normal is lively enough to suit most of the time. The engine revs still shoot upwards in typical CVT fashion when you accelerate, but not as much as before and the rev rise is now matched by a pace rise. The engine feels properly connected to the wheels.
This connectedness continues with the steering, which is precise, quick and credibly weighted. It's the gateway to tidy, fluent handling that even allows a touch of line-tightening when you throttle-off. The Prius is stable in a crosswind, it rides with well-damped suppleness and it cruises quietly. And the transition from regenerative to friction braking is imperceptible.
Should I buy one?
You probably should. It will be offered in four trim levels: Active, Business Edition, Business Edition Plus and Excel, starting at £23,295 and peaking at £27,450. Sat nav is standard on the upper two, while a wireless phone charger and the expected suite of today's safety gear and self-parking systems also inhabit the equipment and price lists. The upper two have 17in wheels and a 6g/km CO2 penalty, but you can insist on the 15in wheels and get £400 back