If you're not filling up at the same pump or at least averaging the
mileage over several fill-ups, you may get abnormal readings, since the
shut-off setting on pumps varies considerably. Additionally, if a trip
is one way, it can be affected by wind and terrain. 40 mpg is beyond
what the Elantra is capable of averaging at normal highway speeds,
though people who drive long distances at ~50 mph have seen that kind of
mileage.

........

..........

Well, the one true trip I took that needed a fill-up off the interstate was
right at 40 mpg.  And the Scangauge in instant mileage mode hovers right at
that rate on level ground (plenty where I live) at 65 - 70. Any faster and
it heads down into high 30 figures you note. So it seems to be fairly
accurate so far, albeit based on that one trip.

I admit my verbiage incompetence.  -Depressingly- low enough to make me
look at the Prius web pages in envy.  And halfway seriousness.  Double city
mileage is serious stuff (to me).  Long term payoff or not.

With that little gas used, trip computers are not very accurate. It
could easily be off by several mpg in either direction.

Shutting off your engine at lights will help significantly.

40? Not likely. Something in the range of 36, perhaps 37 is more like
it, provided that you drive reasonably.

What's impressive about low mileage?

Well, the Scangauge I bought is working great, and I thought I'd share the
new tank of gas mileage with the group.

In -pure- city driving, and we're talking not any free speed length of more
than a quarter mile, ninety percent on stop and go with lights and
stopsigns every two or three blocks.  But decent slow driving.

At this point with close to half a tank used, 22.5 mpg.  And still very
slowly dropping.

Frowny face.

Normally it's a mixture of freeway and town, which puts it around 26 to 27,
but this stint of stop and go really takes it's toll.  The good part is
that I know it will get around 40 on the road, which makes it a little
better.

But still impressively low I think.