WHAT SPEEDO/TRANS GEAR DO I NEED

If the car drives and has a working speedometer, you can calculate the change needed in percentage, from what the Speed-O says to What it Should say.
Then you just add or subtract that percentage to the current gear.
If the speed-O is slow, then it needs to spin faster, so you would subtract teeth, to speed it up.
This method is as accurate as you can maintain your speed to a known standard, and the faster the better.
Example
say you are traveling at a true groundspeed of 60 mph but your speed-o is saying 66mph, so is 10% fast, so you need to slow it down 10%, so you need MORE teeth.
If your gear has 34 teeth, then you need 1.1 x 34= 37.4 teeth, this rounds to 37, and the speed-O will read a hair fast.