Going With The Flow

I commented back in October that I thought raising the motorway speed limit to 80mph would be of little benefit. However, I also commented that that there was little to be gained by driving at 70 over 60mph but that I drive at the higher speed to prevent boredom. I have been down in Liverpool this weekend and, as an experiment, I have actually tried this to see if there is much of a saving to be made by driving more slowly. The aim was to drive down at a steady 70 and back at 60. As I don’t have cruise control I was trying to do this manually and simply speeding up when the speed was below the target speed and slowing down when the speed crept up. It’s a simple enough method but I used the car’s trip computer to calculate average mpg and speed for the journey. Even though this was meant to be a real world test I didn’t exactly have laboratory conditions as when I drove down the weather was clear and dry - driving back, I had torrential  rain and poor visibility: the extra rolling resistance from the wet road as well as the addition of dipped headlights and both sets of wipers permanently on will have had a hit on consumption but I  did, at least have a clear run without delays either way.

The results are quite interesting. Driving down the average speed for the motorway section came in at 66mph which means that the speedometer over-reads by around 4mph. The average fuel consumption for this worked out at 46.5 mpg. On the way back the average speed was 56mph and the fuel consumption 52.5mpg. That’s a reasonable improvement and it is possible that this figure could have been higher if the weather had been better.  The motorway journey took 35 minutes longer and used 2.68 less litres of fuel: about £3.50 worth at current rates. But which was the better journey?

From an environmental point of view, the slower journey was the clear winner: It resulted in 2.68 litres of fuel being unburned which preserves the fuel reserves, reduces the amount of oil needing to be pumped and refined and prevents 6.14 Kg of CO2 being directly emitted from the tail pipe. Additionally, there would be a reduction in wear to tyres and other components with a limited lifespan. Clearly it is a worthwhile exercise environmentally, however, that is not my prime reason for wanting to reduce fuel consumption. Financially, I have saved my £3.50 but it has cost me 35 minutes of my time. This equates to an hourly rate of £6 – considering that this is on taxed income it is more like £9 per hour: well above minimum wage levels but it isn’t that huge that I wouldn’t appreciate a faster journey. I suppose this leads to the next question, which is: is it really worth it?

I don’t particularly enjoy long motorway journeys; I find them rather dull. However, the longer the journey, the more stressful it becomes. This brings up another point which one of my friends pointed out on Sunday night. Driving at roughly 60mph means an awful lot of time sharing road space with HGVs. For the most part they are decent professional drivers but driving near to them is an intimidating experience and there will always be slower vehicles which will necessitate overtaking – this can be quite stressful when the adjacent middle-lane traffic is moving significantly faster. Driving at 70 means that most traffic is travelling at roughly the same speed making overtaking lorries much easier and with only a few vehicles overtaking at higher (and presumably illegal) speeds. My friend’s example was driving down the M6 towards Birmingham. Driving North is easier and the traffic North of Preston is much lighter. When on the M74 it can mean only catching up with a lorry every 5 minutes or so and the middle lane will usually be free at that point. Even so, the whole journey did feel quit a bit more stressful – I suspect it may be easier in a vehicle with cruise control.

It’s an interesting experiment: whilst it clearly shows that travelling at a slower speed will save on fuel, the benefits are not that great to make it the obvious choice for longer journeys. I’d be interested to see the results for similar weather conditions and for the results using a diesel engined car (particularly suited to dull motorway journeys). However, for me the preferable speed is simply to go with the flow. Mind you, I still think that an increase to 80mph is a largely pointless exercise.