Road Type Fuel Economy

A few months back I got myself a new car and one of the features it has is a nifty fuel economy meter.

In my previous car the fuel economy meter gave you an average fuel consumption rating since the last time you rest the meter which could have been up to 6 months ago. The new one does this as well which is good for getting a lifetime average however this isn't the feature I was looking at. As well as the lifetime average car also lets you know your fuel consumption at current moment of driving as well as over the past 30mins with averages shown at 1min intervals. To this extent I was able to test out what sort of consumption I was getting as I was travelling round the country during the break.

For reference, I was using a 2015 Suburu WRX which uses Suburu's new FA20F engine. The engine is a 2.0-litre turbo and puts out 197kW which is about the same as 4.0-litre Ford Falcon. In addition, I was also using 98 octane fuel.

2015 - WRX

Motorway driving

What first made me want to do this testing was when I noticed how low the fuel consumption was on the motorway. 

Note that the lifetime average fuel consumption is the red line through the middle which is also the 8.8l/100km noted at the top. A yellow bars show fuel consumption higher than average and green bars lower than average, each of these bars is the average is taken over a 1min period.

Following are the results from the first test which were taken on the southern motorway. 

The initial yellow spike you can see is from climbing up the Bombay Hills from the Waikato which is a rather steep 8% grade, from there it was a smooth trip all the way through to Ellerslie where there was a night time lane closure in place where I then tuned off the motorway at Greenlane. This trip was finished at 7:03pm and the motorway wasn't all that busy at this time.

Motorway Test - 1

The next image shows a journey from Sylvia Park to the Bombay Service Centre. The two big spikes at the start from the traffic signals along Mt Wellington Highway but from there on its a smooth trip, interestingly the southbound climb up the Bombay hills sits just under the life time average of 8.8 l/100km unlike when heading northbound where the consumption was 15 l/100km. What should be noted on this trip is that it was made in the middle of the day and the motorway was actually really busy yet it was flowing smoothing between 80 & 100 km/h.

Motorway Test 2

Looking at the two trips you could say the average fuel consumption on the motorway is about 6 l/100km which seems like a pretty good rate, amazing given the type of car. I was able to drive the above two trips in a pretty smooth manner, however if there was total flow breakdown on the motorway this value would likely take a beating.

Local Roads

The next obvious thing to test is our local roads given these are the only other option we have within the city. Given it was the summer break it wasn't really the best time to test these as people tend to leave Auckland over summer and the only places you find any traffic is either at the beach or at shopping centres. In any event the following image shows a journey from Mt Eden over to St Heliers, this is a good trip as it goes through a mix of traffic signals and roundabouts.

Referring to the image, the first 4 large spikes are signal controlled intersections with two of them going off the scale. The remaining spikes come from either roundabouts or priority controlled intersections. Generally where you see a green bar this is where I am either going down a hill or slowing down in advance of a red traffic signal.

Local Road Test

As you can see, when driving on local roads the fuel consumption is much more varied then on the motorway with intersections being the killer. From this particular trip you could say the local road fuel consumption was about 8 l/100km however, despite this trip being made in the middle of the day I was almost the only person on the road which made for easy driving and few stops. From previous driving before the summer break my lifetime average was 10 l/100km which was mainly based from city driving and so I think that's a more realistic value when you take account of other users.

Rural Road

The next two images show the fuel consumption on rural roads that aren't expressways or motorways. I've classed the rural roads as two types, 'easy' and 'moderate'. For 'Eesy' you can think of roads such as State Highway 2 heading to Tauranga whereas 'moderate' would be State Highway 1 after the Northern Gateway. 

Easy Rural Road

On the 'easy' rural road we would be looking at an average fuel consumption rating of about 8.5 l/100km,

On the 'moderate' rural road we can see that it really spikes up through the middle which could have been reduced if I eased of the throttle a little. All up however you are probably looking at about 10 l/100km for a moderate section.

Moderate Rural Road

Summary

So in conclusion, using my unscientific but real world test we get the following values for different road types.

• Motorway  =  6.0 l/100km
• Local Road  =  10.0 l/100km
• Rural Road (easy)  =  8.5 l/100km
• Rural Road (moderate)  =  10.0 l/100km

So what does this all mean, well if we were to drive the 655km trip from Auckland to Wellington on an easy rural road we would use 56 litres of fuel, however if this was to be an expressway we would only use 39 litres. That's a grand saving of about $37.4 assuming $2.20 / l.

For a more relevant example we can take the 35km Puhoi to Wellsford project, currently this would use about 3.5l, however as an expressway this would reduce to 2.1l saving you about $3.10 in fuel. If that were the case a $4 toll wouldn't seem like too bad of a deal.

When it comes to fuel efficiency one of the main influencing factors is the way you drive, my little test doesn't go into this however for the most part this test is based on calm and smooth driving with gentle braking and acceleration. The 'moderate' rural road results reflect more aggressive driving however I'd argue the different road types induce different driving behaviours and is therefore still somewhat reflective.