a 275/40/19 is a 27 inch tall tire and if you are at 2500rpm at 70mph, you sir have 4.30s gears. Its actually 70.3. Your tires are the wrong size for a Mustang. Are they drag radials?
A 35 series 19inch is 26.5" and a 30 series is 25.5". Stock is 25.68".
Mine for reference:
3.90 gears...