I have a 1990 Mustang GT. The problem is that I have to change gears before every 5 mph, so Im already in 5th before I hit 40 mph. I was told that the rear ending is to big that why I have to change gear so quickly.
I was wonder if there is way to fix this or a part I need to replace?
I'm not sure how this applies to an engine swap (the purpose of this subforum), but I'll answer anyway. Jack up your rear end. Put a mark on a tire, and a mark on your driveshaft. Rotate the driveshaft and count the revolutions compared to the wheel. Figure 3.5 rotations of the driveshaft to one tire rotation would be 3.55 gears, 3.75 rotations would be 3.73 gears, etc. What RPM are you shifting at? There's nothing wrong with being in 5th at 40 MPH, other than needing to downshift if you need to speed up in any sort of hurry.
also i wouldent take it past 5,000 RPM all the time, it will decrease engine life.. maybe the previous owner knows something but it sounds like you have steep gears, you'd probably want 3:08's for highway driving or 3:55's if you stay mostly in town with lower speed limits