Are cars are more fuel-efficient on town roads than on highways?

 By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

01 June 2014

Even though distances are measured in metres, kilometres and the like, it is usually more convenient (and more useful), to quote them in units of time. Very few frequent-fliers know that the distance from Nairobi to London is 6,800km, but almost all will tell you that it is an eight-hour flight.

In astronomy, distances between stars are measured using units of time. The of-quoted light-year is the distance that a beam of light travels in one year. It is equal to about 9.5 trillion kilometres.

Closer home; it is commonly assumed that a flight between Nairobi and Mombasa takes about 45 minutes. But the reality is that the whole journey normally takes about three to four hours. There are no prizes for guessing the reason for this!

It is commonly said that a car consumes less fuel while driving on a highway than when it is in the city/town roads. This conclusion is based on the common method of measuring fuel consumption in units of distance: that is, kilometres per litre or, sometimes litres per 100km.

However, if the consumption rates are converted to litres per hour, a different picture emerges. My car for example averages about 12.5km per litre on the highway. I usually drive at between 100km/h and 120km/h – in the over 30 years that I have been behind the wheel, I have figured out that driving above 120km/h doesn’t gain you any time in your journey!

My mean cruising speed is 110km/h. At this rate, the car covers 110km every hour. But it consumes one litre of fuel after every 12.5km. Therefore, each hour it takes in 8.8L (110km/h divided by 12.5km/L = 8.8L/h).

Now when driving in town, the car does about 8km/L. That looks worse than the highway consumption rate of 12.5km/L. But considering that urban driving is much slower, it would be worthwhile to find out the consumption rate per hour.

I timed myself recently to see how long it takes to drive from my office in Nairobi city centre to my house on a typical day. Since sometime I have to turn off the engine in the traffic jam (when the cops stop cars for more than 5min), I was careful to stop my timer when the engine was off so that I only get actual engine running time.

It takes 27min to cover the 7.5km distance. That’s an average driving speed of 16.7km/h. By the way; the total travel time (including the stoppages) was about 40min – the cops almost doubled the duration of my journey!

Now let’s do a similar analysis to the one above: with a consumption rate of 8km/L, the car needs 2.1L to do 16.7km. In other words, it consumes only 2.1L/h when driving in town – in the rush-hour.

Since 2.1L/h is obviously less than 8.8L/h, it turns out that the car is more fuel-efficient when driving in town than on the highway.