The Extra Mile

Extra Mile

It takes three times as much energy, and therefore fuel, to drive at a steady 70mph compared with 35mph! Is that shocking? Maybe not, but since only around one third of our fuel is converted into motive energy, it means that we waste three times as much fuel at the higher speed compared with the lower velocity. Are we proposing then that you should drive everywhere at 35mph? No, in fact, it would be wonderful for many urban drivers if they could drive everywhere at a steady 35mph, because it would be a higher average speed than many manage to achieve in their everyday motoring, driving to work, to the shops, for social gatherings, or visiting relatives – and it isn’t getting any better! So why then do we need cars with ridiculous performance potential, like Audi e-tron Sportbacks that do 0-62mph in 5.7 seconds, or the MINI Electric that does the same trip in 7.3 seconds. In real terms, with no gear changes, it’s as fast as any Cooper S. Patently we don’t need that kind of performance in bread and butter hatchbacks, and it destroys the image of EV eco-friendliness. You get to recover less than half of the energy used in accelerating to such speeds so quickly, and it’s wasted energy in anyone’s language, and using it on public roads is anti-social. Plus, it obviously increases the hidden pollution of particulate emissions generated from accelerated tyre wear.


Then again, how much does the performance potential of our car affect our average journey speeds? Very little, I would suggest. Most people probably average something around 60 to 70mph on motorways and fast dual carriageways, although some choose to cruise at lower speeds. It should be possible to drive comfortably at a steady 60 to 65mph on such roads, but it’s not, because the speed differential between you and most HGVs at that speed is small enough to make it difficult, stressful, and even dangerous to mix it with them. So most of us probably cruise at 65 to 75mph. But do we need to raise the maximum speed to 80mph, to increase that speed differential, which some feel would make things safer? Well, the big news for many motorway users is that our Government has just decided, in its wisdom, to increase the speed limit on motorways where roadworks are in operation from 50 to 60mph. An example of the time savings involved (from the official pilot scheme) quotes a saving of 68 seconds over the 13 miles between M1 Junctions 13 and 16. Good news maybe? We think it will be approved of, but why has it taken so long?


Might we now expand those thoughts and findings to consider raising the national speed limit by 10mph, from 70 to 80mph – something that’s been debated over many decades, during which the performance, fuel economy, and safety ratings of our cars has significantly grown? Should the motorway maximum speed be re-examined and a decision taken, at long last? What might be the consequences, emissions and fuel economy-wise? Tests on a Ford Focus diesel, not too many years ago, produced a drop of 9mpg, from around 52mpg to 43mpg, when cruising at 80mph compared with 70mph. That’s around 2p a mile extra in fuel costs, and it means that you then have to stop for fuel more often, losing, say, a quarter of an hour every 500 miles. Of the 50 minutes that you save over 500 miles cruising at 80mph, 15 minutes are lost by having to fill up earlier – if you only take 15 minutes! Your 500 miles is now costing you £10 extra (the cost of covering 500 miles at 2p a mile more) to save 35 minutes in journey time. That’s equivalent to £17 an hour. Is your time really worth that much, we might ask? Maybe, when driving for business, but isn’t it more sensible on long holidays, and on other leisure trips, to start out 45 minutes earlier, and be kinder to your pocket, and the environment? You don’t have to drive up to the legal limit, and if you choose the right day, or time, to travel, you can avoid times when roads are heavily populated with HGVs.

One Response

  1. Some very sensible points raised by Dr Diesel for the diminishing return of cruising at 80 mph speed compared with 70 mph.
    I wish to see the figures for a similar analysis to compare the vehicle at 70 mph and 60 mph.
    A work colleague with 9 points on the licence for speed offences had a daily drive of 50 miles each way from Kent to outer London by the A20, in fear of another conviction and a ban he dropped his bad habits, his journey time increased by only a few minutes and his savings in fuel costs, fines and insurance were sufficient to cure his folly of ignoring the limits

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