Prius – what is it like on the highway part 1

First long trip

Today was the first long journey in the Prius (still unnamed) taking the dogs to Sydney for their grooming and some shopping for us. The Prius was known for having worse fuel economy on the highway compared to the city, the opposite of every other car I’ve known. The difference was slight but it highlighted that stop-start traffic and low speeds suited a hybrid whereas highway driving at 100-110km/h (62-68mph) required the engine to run continually. A modern turbo diesel would be expected to drink less as its torque helps it keep a steady speed regardless of load.

The official figures according to ADR81/02 for the Toyota Prius Hybrid 1.8L NVW30 are:

  • Urban 3.9 l/100km 72.4MPG(UK) 60.6MPG(US) 25.6km/l
  • Extra-urban 3.7 l/100km 76.4MPG(UK) 63.9MPG(US) 27.0km/l
  • Combined 3.9 l/100km 72.4MPG(UK) 60.6MPG(US) 25.6km/l

So that pattern is more conventional.

The route

Gungahlin to Sydney is about 280km (174 miles). The altitude starts at about 620m above sea level (ASL) rises to 695m near the ACT-NSW border, peaks at 756m a few times and remains above 600m for about 165km of the journey.

There's some hills between Canberra and Sydney

Route profile from Canberra to Sydney, courtesy of Google Earth

Until I had seen this profile I had no idea how gradual the slope was. Not very. Interesting also that one climbs 1553m and drops 2144m in the process. Now, the profile is very important to explaining the fuel economy figures, which I’ll get to in a minute.

The measured trip – out lap

The trip started from filling up with 31.6l of 95RON at BP Marulan, about 114km (71 miles) from home and 638m above sea level. From there to Sydney has 810m of climbs at a maximum of 4.6% and 1418m of descents at a maximum of 5.2%. So that stretch happens to include the steepest climb and descent of the entire Canberra to Sydney journey.

I used the cruise control at 110km/h by GPS (114km/h on the speedo). PWR mode was used to enter from ramps but I used ECO mode for the rest of the time. I tried to anticipate steep hills by accelerating before, but I wasn’t a good judge of momentum so the cruise control kicked in to full power mode to restore speed… not good for economy. I didn’t draft or use hypermiling techniques, but I was smooth. Top speed was a burst at about 120km/h (the de facto speed limit on the Hume Hwy) to make way for a another car. The A/C was low at 21.5°c (71F) against 15°c (59F) outside.

I found that on many descents, I was rolling and recharging the battery using zero fuel and maintaining cruise control speed. (Note: the engine must run at those speeds to avoid a large discrepancy between the road speed and transmission speed. It just does it without using fuel, just as a conventional engine might.) With plenty of chances to recharge, there was plenty of battery for hybrid eco gliding.

The last part of the out lap was an EV drive through the underground car park at Cathedral Street Sydney. (On the return journey I nearly used EV for the whole climb back out of the car park, but the engine started with about 50m to go.)

The trip computer read 160.5km (99.7 miles) 4.0l/100km 70.6MPG(UK) 59.1MPG(US) 25km/l with average speed 92km/h (57.1mph). I was very happy and a bit surprised. (That’s not the 168km from Google Earth as I took a more direct route through the suburbs to drop off the dogs and then to the bookshop.)

The measured trip – return

Conditions were a bit warmer for the return, so the A/C was set to 23°c (73F) against 28°c (82F) outside.

At the M5 entrance at Arncliffe the traffic was banked up. It took 10 minutes to drive (in EV) the 250m from the Marsh Street entrance to finally get onto the M5 itself. Soon after joining the stream the traffic sped to 60km/h! Bad merge genes?

Again I stuck to 110km/h (68mph) for the run back. We took one stop at BP Marulan for the dogs to takes a break and get some Hungry Jacks (Burger King).

Note: I might have been best to fill again at this point and get a reasonable downhill/uphill comparison, but I wanted to get home.

But wait! Oddly enough the elevation profile on the return is different. The climbs total 2064m (c.f. 2144m 4% more) the descents total 1465m (c.f. 1553m 6% more) and the maximum slopes are both 3.9% (c.f. 4.6% climb 5.2% descent). The north- and south-bound lanes are divided and in some cases separated by as much as 300m. Perhaps the grade was engineered to be less severe on the “uphill” journey?Going uphill is easier than going downhill?

Going uphill is easier than going downhill? Not quite

Arriving home the trip totals were 450.1km (279.5 miles) 4.6l/100km 61.4 MPG(UK) 51.4MPG(US) 21.7km/l Ave. 78km/h (48.5mph).

The smoother ride home helps explain why I rarely noticed recharging while descending; the slopes weren’t steep enough to maintain 110km/h without the engine.

Interesting preliminary results. We return to Sydney next week for a few days, so I’ll get another go soon. I leave you with this image as I stopped in my driveway with a Ford F250 truck in the background.