Since my car is an 1991 it has one of those speed sensors for the speedo. I am trying to swap 2.73's in my car for 3.42 and understand with a stock 26 inch tire you need a 17 drive gear and a 45 driven gear. On that popular website everyone on here uses it shows the speed sensor cars need a different specific gear for the speed sensor than a speedo cable, this is fine, the problem is that it says there are two kinds of speed sensors one is for 34 to 39 teeth and the other is for 40 to 45. Does this mean I have to spend another $70 for a speed sensor? I dont even know which one I have stock lol.

 

Yes you will need to buy a new speed sensor too.

 

What is the difference between the two if they both are the same type of sensor?

 

Its similar to the 2-series and 3-series rear end gears. The one for the 40-45 tooth is spaced differently for a differently sized drive gear than the 35-39 tooth one.