Is it an aftermarket gas cap? OBD-II standards actually have a pressure relief valve in the gas cap. If it's a cheapie cap, it may not have the proper mechanism. It really sounds like you got the "big gear" transmission. My metro convertible with a 4.39:1 trans and 15" rims runs about 4300RPM's@70mph. If you encounter a hill, downshifting to 4th won't hurt a bit with the 3.83:1 trans. 4th gear is probably equal to the original transmission's 5th gear. This transmission is actually a lot better for the motor, because it won't be wound up all the time. It'll help the engine last a little longer. The cam timing gear from 3tech is only about $25. It'll give your car the kick in the butt it needs, and most likely give it much better MPG. The cam+cam gear is about $120 I think.
http://www.teamswift.net/3tech/
These things should make the car run like a 4cyl on the freeway. And make the engine last a lot longer, cause it's not constantly stressing to produce power.
The other transmissions have a bad habit of making the engines tear up piston rings at 100,000 miles, because it's running at 4000-5000 RPM's constantly.