How to compute for your fuel consumption

When we talk about fuel efficiency, you’ll usually find words and acronyms like averages, kilometers per liter (km/L), and even miles per gallon (mpg) if you delve into some foreign sites and sources. Some people swear by fuel efficiency as the main determining factor when buying a car, and there are even some that will justify abysmal fuel figures for the sake of performance and vehicle dynamics. To each his/her own, we guess.

In this article, we’ll talk about how fuel consumption numbers are generated and we’ll also be looking at some terms and give basic formulas on how you can arrive at a more accurate fuel consumption reading for your vehicle depending on your driving style. Warning, there is some math involved here.

Manual Computation

Nowadays, cars have on board computers to do most of the computations for you and display relevant and on-demand information. There’s also a manual way of computing for your actual fuel consumption, and it’s a pretty simple procedure. The next time you fill your tank to the top at a gas station, follow these steps:

  1. Fill your tank to the top
  2. Zero the trip counter
  3. When you next fill up, note the kilometers driven
  4. Fill the tank again and note the number of litres put in
  5. Divide the number of kilometers driven by the amount of fuel added in litres (kilometers per liter)

For example, I fill up my tank to full, reset my trip meter, drive around for a day or two, and I decide to top up my tank again. My trip meter reads 140 kilometers at the gas station, and it takes 20 liters to fill up my tank again. So I divide 140 by 20, which equals 7. In my last drive between my visits to gas stations, my fuel consumption was 7 km/L.

Liters per 100 kilometers

You’ll also notice that some vehicles use liters per 100 kilometers, or l/100km, to represent your fuel consumption. To convert this to kilometers per liter, simply divide 100 by the number of liters next to the “l”. So, if my reading says 6l/100km, that’s 100 divided by 6, which equals to 16.6 kilometers per liter (km/L). Easy, right?

MPG to km/L (and vice versa)

If you happen to stumble upon a vehicle with a miles per gallon (mpg) and want to convert it to kilometers per liter (km/L), simply multiply the mpg value with 0.4251.

  • 25 miles per gallon multiplied by 0.4251 is equal to 10.63 kilometers per liter.

Now if you want to convert km/L to mpg, you multiply the kilometer per liter value to 2.353.

  • 10 kilometers per liter multiplied by 2.353 is equal to 23.53 miles per gallon.

General guidelines

Remember that fuel consumption numbers provided by manufacturers are simply for comparison purposes, and are usually done in strict testing environments and in perfect conditions. Your driving style greatly sways these numbers in either direction, so the actual fuel consumption can vary from driver to driver, and even the conditions in which the vehicle was driven.

In urban congestion, we tend to keep accelerating at high revs because of the stop and go (frequent takeoffs) situation. A car travelling through congested traffic will tend to consume more fuel than one which is travelling at constant reasonable speeds. On the highway, less stops and constant speed can help improve fuel consumption as we rack up the kilometers.

You’ll also find that many cars employ a start-stop system and hybrid technologies to help alleviate wasted fuel in a variety of driving situations. Transmission with wider bands and more gears to choose from are also becoming the norm, keeping our vehicles in the most efficient and optimum rev range. These all help to reduce fuel consumption and save a little bit more gas for those extra kilometers.

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