I notice that my 2012 Sonata mpg is BETTER with Eco OFF than with it ON.
I have repeated the test run and result is the same - 29mpg with Eco ON and 42 mpg with Eco OFF. Mixture of urban and highway. Anybody else? Help!

 

A 13-mpg swing? I find that hard to believe. My guess is you are driving the car harder in Eco mode because it causes the car to feel sluggish. So you are compensating by using more throttle.

 

Could the switch wiring be reversed?

 

No. And even if it was, it wouldn't cause a 13-mpg swing. 1 or 2 mpg probably at most.

 

Dear NovaResource many thanks for your advice and information. I tested it again today on identical routes and got 30 mpg with Eco on and 40 mpg with Eco off. I checked it using the dashboard meter (between the tacho and speedometer) and the Eco display on the Nav screen (car is SE with Navigation etc). I think I am light-footed driver and did not exceed 45 mph on test journeys, nor used excessive acceleration or braking. Spot checks of instant mpg exceeded 50 mpg with Eco off! Gil

 

You can't use the "estimated" MPG feature of the computer. You have to figure out the MPG based on miles traveled divided by gallons used.

 

It's going to be difficult to do that over identical test routes between two fill-ups. I wouldn't like to run it to dry.
Is the computer mpg very inaccurate then?
By the way I am sorry I should have said I did not exceed 55 mph, not 45 mph in my last message.

 

It's probably a decent average across a whole tank of gas, but instant mileage readings are just a guesstimate from the car's ECU.

If you want to truly test the difference between modes, you need to have a test route that's long as possible. Start at a gas station, and fill up until the pump handle shuts off once (don't top off). Run your route, which ideally is a big loop back to the same gas station. Fill up again and note the number of miles driven and the amount of gas used. Then switch fuel economy modes and run the test route again, and fill up at the end of the test and note number of miles and amount of gas used.

Compare the first test to the second, and you're done. The longer your test route the more accurate your results.