I recently drove around 120km to a meet-up with some friends. It was great to catch up properly for the first time in two years, but aside from the partying, I also had an unrelated revelation. On the way there, I was running late. I’m no speed demon, but let’s say I ‘maximised my opportunities’ to go as fast as legally possible. Around 90% of my time was spent at 112km/hr (the UK limit), including on the M25 past Heathrow Airport. Not hitting traffic here during day light hours is like winning the lottery.
At the end of the drive, the dashboard flashed up the vital statistics: 1hr:17m / 120km / 21.5km/L. The average speed was 93km/hr. After an enjoyable evening, I returned home the following day. I took exactly the same route but the traffic was less favourable. It wasn’t stop-start, but my top and average speeds were both lower. The vital statistics were: 1hr:31m / 120km /26.6km/L. The average speed was 80km/hr.
Crunching the numbers, this is a 24% improvement in fuel efficiency for just a 17% increase in time. The volume of fuel used, and thus its cost and CO2 emissions, were about 19% lower. This was a hugely more significant saving than I would have thought for such a small decrease in speed, even in a petrol-hybrid.
Fuel costs are in sharp focus for many at the moment of course, as prices rise rapidly at the pump. This is just one result of Russia’s actions in Ukraine and resulting sanctions. In the UK, the price of standard unleaded has risen by 30% in just three months, reachingUS$2.20/L on 6 June 2022. This price is typical across much of Europe, as well as in many oil-poor nations in Africa, Asia and the Americas where price rises will be hitting evenharder.1
Unfortunately, my more economical driving style, while effective, has not been able to claw back all of the price rise. Slowing down is still the best tool, however, as air resistance is not proportional to speed. It is proportional to the square of speed. It rises more and more the faster and faster you go. At racing car speeds, the effects are profound. Above 300km/hr it is better to think of the car passing through water rather than air.
Other factors that play into air resistance are harder to change. You can’t do much about air density unless you move to the top of a mountain. The two other factors are frontal area (height x width) and drag coefficient (Dc)- a measure of the vehicle’s ‘slipperyness.’ These are both inherent to the vehicle’s design. The petrol hybrid in my example has a fairly large frontal area of 2.6m2,but a decent Dc of 0.28. Its speed / drag curve is shown in Figure 1, along with curves for a brick-like SUV and a sleek electric saloon.
The inefficiency of tall, wide vehicles is clear to see. All other factors being equal, the air resistance experienced by the SUV is approximately 1.8 times greater than the petrol hybrid and 2.4 times that of the electric saloon. So while the SUV can only reach 86km/hr before surpassing 500N of drag, the petrol hybrid can reach 121km/hr and the electric saloon can gallop along at 138km/hr.
While this may point to the electric saloon being the way to go, air resistance doesn’t care what is powering the vehicle. The electric saloon is efficient because it has a Dc of just 0.24, the lowest of any road car, according to the manufacturer. Many other EV models are large and wide, to accommodate large batteries and thus increase range. Is this really the best plan?
Of course air resistance is not the only factor in the cost of running a vehicle. The fuel type, engine capacity, tyre pressures, rolling resistance, weight and so on all affect different models to different extents. Reducing braking and accelerating and changing gear at the right time will help massively too. However, we can all save money... and cut CO2 emissions... just by living life in the slow lane.
Above: Figure 1 - Speed / drag curves
1. www.globalpetrolprices.com/gasoline_prices
2. www.omnicalculator.com/physics/drag-equation
