I've often considered replacing the rear gear set in the Mustang for improved performance. My hesitation is with the amount of improvement I would experience versus how it might negatively impact driving at highway speeds and in first gear.
When driving to the Goodguys show in March, I took the following picture of my gauge cluster. The cluster shows 70 MPH and 2100 RPM. However, my wife was following behind and my speedometer showed 70 MPH when I was going 74 MPH.
The current rear gear ratio is 3.25:1. My rear tires are 26" tall. My transmission has the following gear ratios:
1 - 3.35
2 - 1.99
3 - 1.33
4 - 1.00
5 - 0.68
If I switch to a 3.8:1 rear gear, my RPM at 70 MPH would be 2300 RPM which is not too bad. At 75 which is the speed limit in many places, I'd be at 2500 RPM. However, I'm concerned about what first gear would be like using a 3.80 rear gear.
I could switch to a 3.55:1 rear gear and it would be better all around except the performance increase would not be that great since I'm already running a 3.25 rear gear. At 75 MPH, the 3.55 gears would put me at 2340 RPM.
I calculated these using the following calculator which seems to be one of the better ones I found: