I was returning from a wonderful trip to Mapungubwe National Park yesterday, and hit a terribly boring stretch of the N1, north of Pretoria. To stay awake, I played with the fuel economy gauge on my Honda Jazz. I usually reset this only when I fill the tank. (Average consumption 6.4 l per 100km if I’m driving, 7.1 if sharing the car with my wife.) Every 15 km, I reset the gauge, and changed the way I was driving.

At 120 kph with the air conditioning on, the gauge said the car was using 7.0 litres per 100 km. With the air conditioning off, this fell to 5.6 litres. This confirmed my habit of rarely using the air conditioner except for hot days on the highway. Even then, I switch it off for uphills—a procedure that I have not vetted with any automotive or air conditioning experts, but it makes sense to me. Keeping the air conditioning off, I tried another 15 kms at 110 kph. Consumption fell to 5.3.

Finally, I tried 100 kph. This was the shocker. I was using only 4.3 litres of petrol per 100 kms. I need to try this again to confirm this result, but if true, I could have saved more than 14 litres of petrol by driving at 100 rather than 120 on a hypothethical highway trip of the same distance. That would spare the atmosphere 35 kilograms of carbon dioxide. (Actually, it’s far more than that, because a third of South Africa’s petrol is derived from coal, so it results in more carbon dioxide, but I haven’t figured out how to calculate that yet.)

Yes, the trip would also have taken just under an hour longer each way, but I was having a pleasant time with my son, working on his history project on Mapungubwe. If I had been listening to a book on CD, I would have been in even less of a hurry. I wouldn’t drive 100 on a busy two-lane highway signposted at 120. Traffic would back up and drivers might start overtaking dangerously. But on a multi-lane, divided highway, 100 is a reasonable—and safer—speed. When I first learned to drive in the States, the legal limit on all highways was considerably lower than 100 kph. (55 miles per hour.)

Try this experiment—or a variation of it—on your own car, if it has a consumption guage. Let us know the results by clicking on “comments” below.