the rev's on my car is about 2500 steady at 75mph.... Is it because of the 5 speed thing? I seen some cars go at 3500 at 75mph....but 4-speed...

In other words, what are the differences, if same horsepower etc...?

 

Gearing is the difference, and gearing varies by model and year transmission. A manual transmission will typically have a taller final gear than an automatic given the same engine because automatics are less efficient and the engine will struggle more if the gear ratio was the same. Automatics are also more complex and therefore tend to be larger, so it's not as easy to cram a 5sp into automatic housing of the same size. For example, I drive a 92 automatic, but also occasionally a 90 standard (both with basically the same engine). The auto is a 4sp and the manual a 5sp. The 5sp 5th gear is slightly taller than the automatic 4th gear so the automatic does around 300rpm higher at 70mph where it makes more power.