Why distances on the same road change

 By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

02 March 2014

Can the distance between two places change even though it is measured by the same equipment and the same route is taken each time? Yes it can! The reason is that that it depends on the traffic conditions.

When there are many vehicles on the road, one has to drive zigzag, changing lanes while overtaking the slower ones. Since the shortest distance between two places is a straight line, this meandering about is bound to increase the distance…perhaps by up to one per cent.

But Alice Muriithi has notice a significant variation in a fixed route. Her school transport bus uses the same route daily but the distance covered varies from 15km to 17km. She writes: “Is there a way to calculate correct mileage and fuel per kilometre?”

Now a one kilometre variation of on a route that is 16km seems quite large. It is plus or minus 6.25%. Therefore, I suspect that Alice’s bus is not sticking strictly to the same route. Perhaps the driver is making small changes in order to avoid traffic.

But on the question of working out the fuel consumption, the best way to go about it is to get a long-term average; say, one month. Working Monday to Friday, the bus will have been on duty for about 20 days and it will clock about 300km to 350km.

The most accurate method would be to fill the tank and record the mileage reading at the start of the study. The bus is then operated normally and if it needs topping up any time, the quantity added is recorded.

Finally, at the end of the study period, the tank is filled again and the final quantity and mileage reading are recorded. With this data, the fuel consumption rate is calculated as follows.

First, get the total distance travelled; this is equal to the final mileage minus the initial mileage reading. Second, the total fuel consumed is calculated; this is the sum of all top-ups and the final required to fill the tank. Note: the initial fill-up is NOT included – you started with a full-tank and ended with a full-tank.

The last step is to simply divide the total distance by the total fuel consumed to get litres per kilometre.

I have been doing this kind of thing with my car since May last year. On 4th May, I filled the tank and the mileage read 56,301km. I have been recording the litres at each top-up and this week I filled it again. The total consumption over the 10 months comes to 1,281L; the final mileage was 68,130km.

From those numbers, it turns out that the average consumption of my car is 9.23km/L or 10.83L/100km. This is a very “good average” figure because it was taken over a long duration and the car has been driven by different people over different terrains and in different traffic conditions.

But is it a “good consumption” figure? Well, to answer that, I’d have to compare it with another similar car and I don’t know any other person who has been keeping that kind of data.