Heavy fuel usage/high revs

[slick1]

We have a 1.3 L 2015 Yaris and a 2006 1.3 L Yaris (both have a 2NZ-FE engine) and get just under 600 km out of a 42 L tank i.e. ~14 km/L (7.1 L/100 km) average... although they can get as low as ~12 km/L and as high as ~17 km/L depending on how they are driven. We have 200,000 km of fuel data on the 2015 and 370,000 km on the 2006.

350 km (8.3 km/L) out of a tank is very low... although the 1.5 L should logically use slightly more than a 1.3 L engine.

The auto on our Yaris also sometimes drops a gear. There seems to be a certain speed that triggers this gear change. If you avoid that particular speed/load it doesn't seem to do it.

You definitely have a problem with fuel use.

How many km has the car travelled?
Is this a noisy engine or a quiet one?
What do the spark plugs look like?
What is the compression like?
Have you considered doing an oil analysis: It's a good way of identifying many problems.
Is the car using oil?

What does a mechanic suggest?

:-)