Hi Juan.
Ron is right, the Groove if done properly, along with protocols, like killing vac. leaks, applicable stuff like IAC mods and PCV Re-route, HAS to work!
That being said, My somewhat informed opinion (I'm hedging here, I'm not even close to the most experienced one here)is:
Older vehicles like GM throttle body injected and lots of '95-and-older vehicles...
These are the ones w/ dumb computers or none at all. Those ECU's seem less likely to be stubborn or to fight gains.
Of course we are talking about stuff that's 17 years old and older, so you would likely have other issues stemming from age/mileage.
I WILL offer that my first Groove was a '90 or so Jeep cherokee w/ Jeep inline six and port injection. A simple system. Vehicle was a beater hi miles cruddy intake w/carbon and oil crud issues, badly needing a full tuneup. This engine Loved the Groove
power throttle response really improved. Dunno 'bout MPG, was part of neighbor buddy's Mothball fleet don't know if he even has it today. But in general I think those are good rugged reliable vehicles and useful too...
but of course I would do homework check Jeep forums ask around find out more about 'em before I would buy one!
If you DID get one w/Jeep inline six you could shamelessly copy what I did on that TB, it should be in Index Groovy Vehicles, seems I hit the right technique for that one...
Or just get what you would buy anyway and we will help you w/ whatever issues you have. If you buy something after 1996 it is OBD2, a decent scantool is good to have to see what computer is up to for those.
Good Luck and Good Hunting!
Tracy G