I'll start off by saying I've searched and read all the stickys and I can't figure it out. Math is horrible for me anyway so I'll just ask. I've got a 89 Iroc Z with a L98, 700r4 and someone swapped in a 10 bolt for the original 2.77 geared 9 bolt. I'm not sure whats in it but the speedometer is fast. I'm going to be running a 3.42 gear and a 245-50-16 tire. I have no idea what color my speedometer gears are currently but I need to know what gears I need to order to make it accurate. I'm going to be ordering some other stuff from summit and wanted to get it all in one whack. Thanks

 

Here's a GM chart.
Every online calculator will tell you something different than that. I've had excellent results going with the GM speedo gear combinations.

Your tire size's diameter is in between the 2nd and 3rd tire sizes listed for 3.42 gears.

 

This is a no-cal combo, there's no correct calibration possible. As such, best to use a 15-tooth drive and a 40-tooth driven. This will keep you from getting any speeding tickets, unless you are speeding.

 

Ok just to make sure I'm looking at this right I need to go to summit and order a grey 15 and a 41 yellow, correct? Thanks. Is there anyway of knowing whats in there now?

 

No, not correct. Use a 40, NOT a 41.

 

It's really easy to pull the driven speedo gear and count those teeth.
Counting the drive gear teeth inside depends on your determination. A picture might help.