This is a curiosity question. I have a 2010 hybrid but I'm pretty sure that this would apply to other years as well. I don't think that anything is faulty.

Travelling at highway speed (usually around 75 mph), releasing and tapping the accelerator causes the MPG meter to jump up to a higher number. Typically, if it is at 30 - 35, doing this can bump it up to 40+. This isn't pulse and glide as I am not keeping my foot off the accelerator for more than a couple of seconds and I am maintaining a pretty constant speed. The ICE isn't shutting down (logically and as per the info on the dash). The MPG meter seems to stick at the lower number after accelerating or going up a hill (even a shallow incline). It will rise if there is then a downhill section but maybe not as much as it would if I use this procedure.

Since I am releasing the accelerator, it makes sense that the meter would rise initially. Since it is probably averaging, it would also make sense that it would then start dropping as I give it enough gas to maintain my speed. The fact is however; that it does not drop back to where it was previously but will stay at 40 or better (on a flat road). What does this actually do that causes the MPG to rise and stay at a higher number?

 

Most of the time, it is a result of a slightly less input on the throttle. Most people will hold the throttle at the position it took to get to a certain speed but you can actually back of just a hair and maintain speed but you mileage will get a boost.


The other way this can happen (and what I think yours is doing) is that some energy is being used from the batteries to give the ICE a boost with the electric motor. I can often play with mine on the interstate and get quite a bit more mileage for several miles doing this.