My 1.6 litre '99 Sentra was a slug, but now it's fun.
I've been running Tufoil in the engine for quite a while. I added a Tornado fuel saver to the air line. The Tornado gave me more power with the a/c on than I had before with it off. I think it makes the fuel-air mixture atomize better.
On a recent 1900 mile trip with some city driving I got 43 mpg. I only go 50-55, though.
Just passing on the info. I am not associated with the Tufoil or Tornado companies.
There is one drawback -- it comes on too strong when I first touch the gas pedal. In 3rd trying to go 25 mph, I have to actually get out of the gas to keep it from going over.
Tufoil does make the engine last longer too. Possibly 40%.
Get ready for a flame war ... but both products are refundable.