Rear gear in an 89 mustang?

Jack the back up and pull the diff cover (probably need to change gear fluid anyway) ..... count the teeth on the ring gear and divide by the number of teeth on the pinion.

Example: 39/11 = 3.545 = 3.55 41/11 = 3.727 = 3.73

that works, or, if your considering a gear swap and wanna know whats in there before you pop the cover jack the back end up, BOTH rear wheels off the ground, mark the driveshaft and rotate one of the rear wheels one complete turn and count how many times the drive shaft spins. if it turns almost 2 3/4's you have 2.73, of it turns about 3 times 3.08's, 3 and 1/2 times 3,55, well you get the picture. i wouldnt go over 3.55 if you like burying your speedo. it depends what you have in there already....

if the car is original most likely they are 2:73's.

3:55's are a good daily driver gear, possibly 3:73's.

with my 18" rims and 3:55's i do just over 2 grand on the freeway at 70 +/- however far off the speedo is.

Go with the 3.73's. I commute 70 miles per day, and have no problems with my 3.73's. Gas mileage at 80-85 mph freeway is right around 20, and the rpms arent to extreme.

3.55 gears
Cruse on the highway at 65 MPH in 5th gear
Engine RPM: 2057
Vehicle Speed: 65

4th gear
Engine RPM: 5200
Vehicle Speed: 111
Read End Gear: 3.55


3.73 gears
Cruse on the highway at 65 MPH in 5th gear
Engine RPM: 2162.
Vehicle Speed: 65

4th gear
Engine RPM: 5200
Vehicle Speed: 106.
Read End Gear: 3.73

As you can see, not much difference. Most stock 5.0's start to run out of power above 4700 RPM, so with a stock engine 3.55's would be better for the strip. They would put you in the peak power range as you cross the 1/4 mile. Any bigger gear would mean that you really should upshift because the engine power has started to drop off.