pgupta0

I just had 10,000 mile service performed on my 2014 ES300H. I mentioned to the service manager that my mileage has come down to 36-38 mpg from 39-41 earlier. He advised me not to rely on the car display reading but do it manually for more accuracy. He said that many owners have found that they get better mileage than shown by the display.
My question to you is whether you have experienced this sort of consistent error in your 300H?

jollick

I have had my 2013 since new in January 2013. I always calculate my MPG and have found the on board calculation is usually off by 1 to 2 MPG. The on board calculation is showing higher than manually calculated. Driving normally but coasting to red lights I get 37 mpg to 38 in the warm months and 35 mpg to 36 in the cold months. In the cold months the the engine has to run when first started to get the catalytic converter up to operating temperature and that waists gas. You can also turn off the heat while sitting at a light and notice the engine stop since it is not running to supply it. My area has a lot of hills and that hurts my MPG a bit. Also the hybrid battery is not as efficient when it is cold out.

PirelliRC

I just went through my first tank and when i calculated the totals by myself it was 1.7 mpg less than what the on board indicator was displaying. So about 1-2 miles that jollick suggested seems to be accurate. Its definitely no big deal seeing that my challenger displays around 4 mpg more than what im actually getting. I dont understand how car makers cannot be more accurate. Its simple math.

chromedome

I wonder how the ECU calculates fuel effiency. Does it do some fancy calculus with the distance traveled and some lookup tables listing fuel consumption at specific speeds/grades/temperatures? Or does it actually monitor precise fuel flow to the injectors and correlates that with driven distance?

wowzer

My first tank it seem to over estimate the efficiency. On my Prius, it overestimated as well. Overall, the computer seems to rate higher efficiency than what you're really getting.

SLH

Probably, neither of those. The ECU controls the fuel injectors, so it should not need to monitor or measure the flow. That is, the Engine Controller operates the fuel injectors, telling them when to open and close, and so it controls how much time they spend open and how much fuel they should be injecting. Keeping track of how much time they are open, and knowing the flow rate, should be enough to calculate the volume of fuel used.

Variations in operating speed of the solenoid valves, and in the flow rate, due to component manufacturing tolerance or wear during operation, are the most likely sources of any inaccuracy.