Whenever you change transmission, rear end gears or even rear tire size, the speedometer may read inaccurate.

It is impossible to calculate what gear you need unless we know the drive gear number of teeth, rear end ratio and the tire diameter.  If you have all that information, here are a couple of formulas that will help calculate what driven gear you need.

1. Drive gear - You have to count them on your transmission tailshaft.
2. Axle ratio - You must know the axle ratio, and rotating the driveshaft while counting wheel revs might not be very accurate.  Most accurate way to know is to count the ring and pinion teeth and divide the ring by the pinion.
3. Tire revs per mile.  You can look up on www.tirerack.com for your particular tire size, or measure the tire diameter and use this formula to determine revs/mile:    TIRE REVS PER MILE = 20168 divided by TIRE DIAMETER
4. Driven gear tooth count.  This formula calculates the number of teeth required on the driven gear provided you know the previous three numbers:  (DRIVE TEETH x AXLE RATIO x TIRE REVS per MILE) divided by 1001 = DRIVEN GEAR TOOTH COUNT

But, there is a much easier way...  Assuming the car can be driven and has a working speedometer, accurate or not.
All you need to know to get the proper gear is the number of teeth on your existing driven gear on the cable.  It takes five minutes to pull the cable and count the teeth.  Then do this:

There are some smart phone aps that have digital GPS speedometers.  The one I use is called SpeedHUD.  If you can drive your car at an indicated (by the car speedo) 60 MPH, then read out the phone's ACTUAL speed when your car thinks it is going 60, you can divide the two to get a ratio.  Then remove your existing gear and count the teeth.  Use the ratio to calculate how many teeth you need to get a correct reading at 60 mph.

Example...  If car speedo reads faster than the phone
Car speedo reads 60 MPH
Phone reads 50 MPH
60 divided by 50 = 1.2
Multiply the number of teeth on your speedo cable's driven gear by 1.2
So, if you have a 17 tooth driven gear, multiply 17 times 1.2 = 20.4. 
You would need a 20 tooth gear.

If car speedo reads slower than the phone
Car speedo reads 50 MPH
Phone reads 60 MPH
50/60=0.833
Multiply the 17 tooth driven gear by 0.833 to get 14.16
You would need 14 tooth gear.

Just use those formulas and insert your own numbers to figure the % change you need to make to your existing gear.  This works perfectly every time.