How far can you go on a tank?
My latest graph based obsession is fuel economy. After doing a film for Watchdog on it a few months ago (based on the cheap petrol guide), I wanted to see if I could further drive down my costs with a simple challenge: how far can I go on a tank?
This isn’t my first foray into petrol cost cracking. A few years ago, when I proposed to the now Mrs. MSE on holiday, I undertook a fuel efficiency experiment (yes I know, I’m such a romantic), and boosted it by around 20% (see the petrol efficiency experiment blog).
Recently I wanted to take this a step further. I’ve been driving in a fuel efficient way, as you’d expect, for a good time, and I have a smart car with a 699cc engine (one of the small two door ones), which is perfect for central London driving.
- The experiment. The measurement is easy, once my car is low on petrol, it indicates the number of litres left, so I can be pretty specific about how much I put in. I refill it when there’s only 4.5 litres left, and put in the amount until I hear the click and no more.
- How consistent are my journeys? I don’t use the car to go to work (I usually either go on my scooter or I jog), but when I do drive it, it’s in central London, which means serious urban driving, i.e. very low mpg even in a Smart, with all that sitting in traffic or moving very slowly.
The Results.
Here are the results for the last three tanks of petrol:
- Test Tank: Here I drove as I normally do, conscious of my fuel use, and using the right driving tactics, but not overly anal about it. Tank Distance: 245 miles
- Tank 1: Here I started driving with much more attention to fuel efficiency, road positioning at every moment, only pressing the accelerator when needed, trying to stop gradually without using the brakes (see driving techniques in the cheap petrol guide for more), and this improves safety too. Tank Distance: 269 miles.
It should be noted, Mrs MSE borrowed my car at some points during this particular trial, and she wasn’t using the technique to any great extent, much to my ‘experimental’ frustration.
- Tank 2: Continuing with the same technique I found it’d now got much easier. It’s all about forming the habit, and I’m now driving in the super efficient way without having to think about it. Tank Distance: 293 miles
Within this trial, there was one 50 miles round-trip outside London on the motorway, which is likely to have helped a little.
The amount of fuel I usually use is around 30 litres, which costs around £37. So this decreased my cost per mile from 15.1p to 12.6p, a substantial saving. And as I say I was already driving pretty efficiently, though without my current obsessional methods.
A few super nerdy observations
It's not about speed
- Stay a good way behind the car in front. On one section of a journey, with busy traffic moving at about 20 miles an hour, I counted how many times the car in front of me braked at lights compared to me. It braked 27 times over a 2 mile stretch, but I didn’t brake at all. I had given myself just enough space that when it braked, I could just stop accelerating and slow down gradually.
While you may not think braking has anything to do with saving petrol, remember when you brake you are getting rid of the energy that you’ve paid to build up in the car, therefore the less you need to brake, the more efficient your driving is (though safety first obviously).
- It’s not about speed it’s about acceleration.
I can’t say this one enough, I’m not driving more slowly, I’m just taking a few seconds longer to get to the same speed, and very rarely does that make any difference to the time of the journey (in urban driving it just tends to mean you catch up with the car in front of you at the traffic light).Of course there is a balance of consideration here, you don’t want to annoy the driver behind by accelerating too slowly. Yet I think it is possible to find a happy medium.
- The traffic light dilemma. The one dilemma I have is when closing in on a traffic light. The most efficient way to travel through is not to stop, i.e. go through on green, yet sometimes the only way to ensure you make it in time is to accelerate more quickly from the last one (especially if you know the sequence).
So there are times I did find myself using the accelerator a bit more to save petrol.
I would love to know how you get on if you try it?
