Don't believe what you read on Ebay, as most of it is misleading at best
and much of it is just plain lies. While installing a CAI may add
SLIGHTLY to your Elantra's horsepower, it would be more in the realm of
1 or 2 (if any) than 10. It may also increase fuel efficiency slight,
perhaps 1 or 2 mpg, but that's not a given, either. More than anything
else, they change the sound of the engine, which is what some people
want, but it's basically like putting a dress on a pig.

The chips sold on Ebay are junk and they don't work. There is a
replacement ECU chip called a Powerchip that will increase performance,
but it's expensive and it requires that you use premium fuel to get the
most out of it.

Switching to synthetic oils in the engine and (manual) transmission can
help fuel economy, especially if you live in an area where it gets cold
in the winter. Synthetic engine oils remain thin at cold temps, reducing
drag in the engine as it warms up. A synthetic tranny oil like Redline's
MT-90 improves shifting, especially again in cold weather, since it
doesn't become as thick as molasses.