Car speed and efficiency

Call me somewhat obsessed if you like, but ever since acquiring a car (2006 Renault Scenic) with an actual computer inside, along with a miles-per-gallon readout, I've been taking rather an interest in the efficiency at various road speeds. Yes, I realise that petrol has become cheaper in the last month or so, but it was downright pricey for most of 2014, so every gallon saved is the best part of £10 back in my wallet.

So what IS the most efficient speed to drive along at? Classic car manuals and Internet advice often throw up '56mph', but then we've seen that speed quoted from as early as the 1950's, when car shape and aerodynamics were incredibly primitive. Surely modern cars, designed in wind tunnels, should be able to slice through the air better and this should have some effect on fuel efficiency?

[The problem with going fast is that there is far more air resistance - this increases as the square of velocity, so there's four times as much drag to overcome at 70mph than at 35mph, and so on. Physics, eh?!]

Of course, there are a mountain of other factors that affect both the most efficient speed and also day to day testing, not least:

  • car shape
  • load
  • tyre inflation
  • weather, including head and tailwinds
  • driving and traffic conditions
  • time of day, i.e. whether headlights, fog lights, internal heating fans, etc. were on
  • driving style (especially up hills)
  • engine and gearbox condition and capabilities

That's a lot of variables, so take my comments below with a pinch of salt.


But the question remained - at what speed should I drive, in my Scenic, at the start of 2015 in the UK, for best results?

With this in mind, and with some lengthy journeys back and forwards across the UK needed (ailing parents), I set about logging approximate fuel economy while trying to keep to certain speeds, with these estimates gathered over around 2000 miles:

  • At 70mph - around 41mpg
  • At 65mph - around 44mpg
  • At 60mph - around 42mpg

In other words, I contend, the sweet spot for modern (post 2000) cars is higher, possibly as much as 10mph higher, than it was for cars back in the 1950's, 60's and 70's, when the classic '56mph' was coined. I've not seen anyone else update this figure formally, even online, but both my own data and common sense suggests that it's true.

A sleeker, better shaped modern car, with engine and gearbox designed to ride easily at 3000rpm and up to 70mph, is worlds away from (e.g.) the Maxis, Capris and Escorts of my youth (in the 1970's).

So here's to, reversing the digits rather memorably, 65mph as the new optimum, most efficient speed to drive at in the modern age.

PS. Of course, there are other parts to the overall equation, namely that time is money for many people and the more precious resource. So it may well be worth spending the extra on fuel at 70mph or 75mph, for example, in order to arrive quicker 8-)

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