Speedometer off...

In the past what I did was:
Take the car out on the highway note the odo setting at any mile marker. then drive 10 miles and note the odo setting again.

if your odo says you went 13 miles then your speedo cable is turning 30% too fast so you need 30% more teeth in your trans gear. so 33 * 1.3 = 43

if your odo says you went 8 miles then your speedo cable is not turning fast enough so you need 20 % less teeth.
or .8 * 33 = 26 roughly.

so you don't need to know your gear ratio or tire size etc. just the size of your current gear.

If your odo is reading correctly then your speedo will also.

The reason you need to go 10 miles is to eliminate the error in reading your odo. +- 5/100