How fuel consumption is calculated

Fuel consumption shown on the windscreens of cars, and often repeated in sales brochures, have been criticised for being too optimistic.

This article describes the European Urban Driving Cycle (UDC) and the Extra-Urban Driving Cycle (EUDC) for cars equipped with a manual gearbox on, which these fuel consumption claims are often based.

These tests are actually based on a typical short journey in Paris, and it will become obvious that the average speeds are so low that the results certainly cannot apply to the way most people use a car.

The UDC test cycle

The vehicle is placed in a closed test cell where the temperature and pressure can be controlled.

Wheels are placed on rollers of a special chassis dynamometer able to simulate the resistance the car will feel on a road.

Temperature is between 20 and 30 degrees Celsius and all the extraneous engine loads, such as air conditioner, lights and heated rear window, are turned off.

A driver takes the car through its paces.

Each portion of the test consists of idling, accelerating, cruising or slowing down in a timed sequence.

At zero seconds the car has to: idle for 11sec, slowly accelerate to 15km/h in 4sec, cruise at this speed for 8sec, brake to a stop in 5sec and idle for 21sec.

At 49sec the car has to: slowly accelerate to 32 km/h in 12sec, cruise at this speed for 24sec, slowly brake to a stop in 11sec and pause for another 21sec.

At 117sec the car has to: slowly accelerate to 50km/h in 26sec, cruise at this speed for 12sec, decelerate to 35km/h in 8sec, cruise for another 13sec, brake to a stop in 12sec and pause for 7sec in neutral.

The cycle ends at 195sec after a theoretical distance of 1 017m.

It is repeated three times.

The total duration is 13min over a theoretical distance of 4 067m at an average speed of 18.77 km/h.

The EUDC test cycle

Complaints that the UDC test is not severe enough have led to the introduction of the Extra-urban Driving Cycle, tacked-on to the end of the above cycle.

When the UDC test is completed, the driver has to: wait for 20sec in 1st gear with the clutch pedal down, slowly accelerate to 70km/h in 41sec, cruise for 50sec in 5th gear, decelerate to 50km/h in 8sec, cruise for 69sec and slowly accelerate to 70km/h in 13sec.

At 201sec he has to: cruise at 70km/h for 50sec in 5th gear, slowly accelerate to 100km/h in 35sec and cruise for 30sec in 5th or 6th gear.

At 316sec he has to: slowly accelerates to 120km/h in 20sec, cruise for 10sec, slowly come to a stop in 34sec and let the engine idle for 20sec in neutral.

The total duration of the EUDC test is 6min 40 sec, and the theoretical distance is 6 956km with an average speed of 62,6km/h.

The calculated fuel consumption in litres/100 km as well as the equivalent CO2 values in g/km is based on the combined results of the two tests.

The total time taken is 20min 33sec, at an average speed of 33.6km/h over 11.023 km.