I had another opportunity to gauge the Prius’ highway skills this week to spend a few days in Sydney.
Filled up in Canberra with 31l of 98RON (to dilute the E10 further) before leaving to get a full journey reading. I can record fuel purchases in my Garmin 2460LT, which told me that my mileage to that point was 4.9l/100km 57.6 mpg(UK) 48.2 mpg(US) 20.4km/l. This will take a little while to settle down as it is hard to filled the tank to the same point every time. The figure is slightly greater than the HSI (Hybrid System Indicator) suggests.
Run from Canberra to Sydney measured 279.1km, averaged 86km/h and drank 4.3l/100km 65.7 mpg(UK) 54.9 mpg(US) 23.3km/l. Conditions were slightly warmer to last weekend and again I set the cruise control to 114km (110km by GPS) and didn’t drive in any special way. However, I was carrying luggage and a heavy portable air conditioner in the boot. That indicates that the 4.0l/100km figure I got from nearly the top of the hills as it were to Sydney is only slightly affected by the previous 125km of climbs and descents.
The only trip by car in Sydney was to my sister’s house in Turramurra. Again the elevation profiles to and from are quite distinct, however the bulk of the distance was on opposite sides of the same road and therefore the same profile. Traffic was heavy on the way up, so the start-stop engine came in handy. Light traffic and lots of long downhills helped on the way back. The 50km took the HSI to 4.9l/100km 57.65 mpg(UK) 48.23 mpg(US) 20.41km/l overall at 231.2km.
The way North goes via Oxford Street and Hyde Park to the Sydney Harbour Tunnel. Curiously the minimum altitude is supposedly 4m ASL even though the tunnel reaches 25m BSL. The drop into and climb out of the tunnel are the steepest long sections. There are steeper parts including a 25.9% climb from Woolloomooloo, but these are very short.
The return journey took the Harbour Bridge. The elevation profile below is for the Cahill Expressway, however it was closed for roadwork that night. The detour took the Western Distributor and Bathurst Street. Again the minimum altitude of 4m was centred on the Sydney Harbour Bridge, which is odd because I used to calibrate the altitude on my watch to 60m when I took the bus it each morning. The real figure should peak at 59m ASL.
Return journey (less a portable A/C, plus lots of shopping) was mild with some heavy rain in parts. Lots of roadwork involving 80km/h and even 40km/h stretches, which were good for charging the battery and using little petrol, but strung a bit on accelerating back to 110km/h.
The final figures were 649.6km (403 miles) averaging 73km/h (45.3 mph) drinking 4.6l/100km 61.4 mpg(UK) 51.4 mpg(US) 21.7km/l.
All this tells me a few things:
- Google Earth elevation profile feature is not perfect and depends heavily on the zoom level at which it is displayed;
- HSI figures may vary from actual tank-to-tank measurements;
- You don’t need to drive the Prius in a special way to get good economy figures. Even driving on cruise control, using A/C and steep hills doesn’t double its consumption;
- 4.6l/100km is a reasonable estimate of fuel use on that run under a variety of conditions.
Yes, my sample size might only be two, but I think that I have a reasonable benchmark as a starting point.