This has been brought up before, and I have even noticed 2 mpg less than what my trip computer says in the past, but just the opposite happened today. When I filled up, the trip computer said 25.4 mpg but I calculated 25.8 mpg! I did top the tank off a little bit, and the car was level, so I'm sure I got a full tank of gas.

Two things may have changed from my previous fill-ups. The first being that on this last tank, I didn't do any coasting out of gear while stopping (the car will coast forever). I just coasted in gear. My thought is that maybe being out of gear caused the trip computer to give unrealistic mileage estimates (reset your TC while coasting and watch 99.9 MPG be displayed). Previously, I've been taking it out of gear and coasting to try and boost mileage before coming to a stop. The other thing that may be a factor is now I have over 3000 miles and it is getting broken in. I have also stopped babying it so much and this last tank I drove more aggressively than I had been and my mileage did not change!!! Anyone else notice any changes like this?

 

For some reason this thread double posted and we don't need two of these. Can somebody delete one of them?

 

just exactly how in the world do you calculate gas mileage? i've always wanted to know without depending on the trip calculator.

 

Miles from trip odomoter/gallons put in tank (thus miles per gallon)

 

I think most likely at those moments the car REALLY IS getting that rate of efficiency.

I've noted before that we haven't "honored" the Trip Computer sufficiently for how much of a learning tool and even a research tool it is. I've concluded that the seemingly-odd extreme numbers that we get (both high and low) are probably right. For example, besides that cool "99.9" that we sometimes get, we see that when we're just coasting on a slight downgrade, the mpg isn't what we might have guessed, which I think would be maybe 40 -- it's more like 50 to 60. And on the other hand, when you floor it, you use about half of the tank. Also, we see that the A/C, at least on this car, has very little effect on mpg. In short, road conditions and how your drive can have extreme effects on mpg, much more than probably hardly any of us thought.

I drive very differently during different periods of time, depending on a lot of things. And I've kept a pretty close watch on how the moment-to-moment display of mpg correlates with the "average" mpg that is shown, as well as how both of these relate to the actual mpg that I can calculate the "old-fashioned" way.

And I'm convinced that all of what you see on the Trip Computer, including those weird extreme momentary mpg figures, are very accurate. Not exact, but essentially correct.