my prius in highway srilanka 120km/h

welcome and congrats, that's excellent performance!

 

	metric	convert	SAE	convert	metric
1	120km/h	x/1.6	75 mph	x*1.6	120 km/h
2	20km/L	x*2.352	47.04 MPG	x/2.352	20km/L

So these conversions and the following table provides a rule-of-thumb:

Of course it is possible to generate the same table using standard (aka., metric) units. There is an air temperature effect that is proportional to the absolute temperature, K, and a constant. In aviation, we call it the density altitude, the degree that air density changes as a function of altitude and temperature. You can see this as the dip in MPG at 80 mph when the air was 62F versus 82-86F.

As a general rule, density altitude effects do not become significant until temperatures are in the 60F (15C) and below. Empirical testing suggests 85F (30C) is a peak efficiency temperature but it also depends upon when the temperature requires air conditioning.

One heads up, the car takes about 5 minutes to warm-up. During this time, it gets ordinary mileage as fuel is burned to bring the engine, transmission, and tires to operating temperature. So the rule of thumb is:

• slowest practical speeds for first 5 minutes - often taking a route through a neighborhood instead of directly to the cross-town route.
• drive the car normally but let the car be 'stuck' behind slower traffic - do not tailgate but use large, slow vehicles as a pacing car. Following traffic will happily pass the 'poor Prius' stuck behind the truck and never give it another thought.
• if driven to work, drive to lunch - this will 'reheat' the car, often at slow speeds, and make the evening commute more efficient. Fill the seats with co-workers and just keep the fuel consumption display showing, not the others. <grins>
  • If you have co-workers returning from lunch and stuck at the head of a traffic light queue, let the engine stop by itself and shift into "N". Then holding the accelerator down, suggest 'we need to get back to work' and when the light turns green, shift into "D". After all, we don't want our slow, fuel efficient driving give the wrong impression.

This is such a great car,
Bob Wilson