View Full Version : 2010 Forester MPG calculation

David Mansfield
11-15-2010, 07:49 PM
I have a 30 day old 2010 Forester. It has the MPG read out on the dash. How is that figure calculated?

I have read the manual but it does not explain the calculation. Obviously the trip odometer is the numerator but what is the denomonator and how is it determined?

11-16-2010, 12:27 PM
There is a computer algorithm in all of these gauges. They are unique to each vehicle. They are almost always optimisitic. My 09 Ltd is usually .7 mpg high but can be 1.5 mpg high. The Hondas are far more optimistic. A friend who owns a CRV says that her car registers a consistent 33 mpg at 70 mph.

David Mansfield
11-17-2010, 10:26 AM
Thanks for your reply.

I understand that there is an algorithm in the vehicle's computer. That was my question, how is the MPG calculated or, stated another way, what is this algorithm?

11-17-2010, 07:13 PM
It's a VERY simple algorythm in theory (manufacturers may complicate it unnecessarily in their programming).


So if you travel 30 miles, and it takes you 1 gallon of fuel, you are getting 30mpg.

Is that what you wanted to know?

David Mansfield
11-18-2010, 11:08 AM
Ah hah, why didn't I think of that! Just kidding.

We are getting closer to answering my question, where does the computer get the information for the gallons of fuel used?

Does it measure the level change in the gas tank? I don't know of any fuel gauge that is that accurate.

Is there an inline fuel flow instrument? I don't know, if there is it would be an expensive item.

Is it a complex measurement of engine vacuum, gear use, tachometer reading, engine temperature all factors in fuel consumption. ... ??

Some body has to be "smarter that the average bear" and know the answer.