Difference between displayed and actual MPG

Speedometer may be off, but odometer should be accurate. The latter is what the car would be using to calculate mpg.

 

If the speedometer is misrepresenting speed of travel, how is the odometer going to be accurate? Speed x time = distance traveled = odometer reading?

As stated earlier, we work with the data we've got.

 

If you hook up something like a Scangauge to the OBD port, and watch speed, you’ll see a speed lower than the speed displayed by the speedometer. The odometer is roughly accurate, subject to vagaries like tire wear. If it wasn’t Toyota would get into legal troubles.

In a nutshell, the car has a fairly accurate perception of:

1. Distance travelled
2. Speed
3. Fuel economy

It leaves #1 as-is.

It makes an adjustment to #2, due to regulations, to err on the highside, for the purpose never understating speed, slowing drivers down some.

It makes an adjustment to #3, for reason known only to themselves, to err on the "rosy" side, but their motivation is pretty obvious.

 

Short drives are always worse than long. Our commute is typically 40 miles each way and mpg is fairly accurate.

 

I am aware of the historic bias in odometer/speedometer readings, though I wasn’t sure that continued into the present era. But if so that would tend to decrease rather than increase the discrepancy between the two measures we are here discussing; in other words it would make the bias programmed into the computer by Toyota engineers even worse.
The odometer reading high wouldn’t explain the discrepancy we are seeing but rather suggest the opposite.

Edit: Mendel Leisk said it better than I did a few posts up. I missed his reply.

 

I simply prefer raw unbiased/unfiltered data rather than data that has been 'adjusted/corrected' - that simply introduces bias and errors. Sorry, just an old fart....

 

Well of course. Who wouldn’t?
The point of this topic, this conversation, though, is how to make sense of the very much massaged, tweaked, adjusted, flattering, misleading output of the fuel economy computer built into these cars.

 

We're back to; "we work with the data we have".

 

Our 06 civic hybrid had a mpg display that was either spot-on, or slightly pessimistic.

I've heard too, I think it was with the advent of 4th gen, Wayne Gerdes had a pre-production model of the new Prius, and the mpg was accurate. They "fixed" that when it went into production.

Again, at least from reports here, with gen 5 Toyota seems to have left the mpg display accurate.

 

Super interesting! I appreciate all the good insights you have to offer on this subject.