how can you tell what size gears you have?
 

I have the problem with my speedo as most people here, I have bigger gears in the diff, but dont have the speedo gear to let my speedo know that I have bigger gears. I want to buy the right gear for my speedo, but since I bought my car after the gears were installed, I don't know what size the gears are. I'm pretty sure they are 3.73, but I dont want to buy the wrong gear. Is there any way of being able to tell, and if so how?

well there are a couple of ways.

1. take your diff cover off and count the teeth on the ring and pinion, and then divide them to come up with the gear ratio.

2. put your car on jack stands with it in gear and make a mark on your driveshaft. Spin one of the rear tires one entire revolution and watch the drive shaft to tell how many revolutions it makes during the one tire revolution. If the driveshaft spins almost four complete times during the tire rotation, then you have 3.73's.

It should say on a tag on your axle.

This works well, but put the tranny in neutral, rather than in gear.

Take care,
-Chris

oops. hehe it's been a while since I did this. Didn't mean to mislead you.

Name that gear
 

There's also a door tag with a code but if you have aftermarket (non-stock) rear gears, all that information is useless.

Use the 'marked driveshaft/one tire rotation' method to determine rear diff ratios, especially on older Mustangs that very often have aftermarket gears installed as this is a basic Mustang mod.

i just got done and the driveshaft passed 3 1/2 times what size grears do i have i would think 355 if four full truns is 373

3.5 rotations = 3.55
3.75 rotations = 3.73
4 rotations = 4.10 or possibly a 3.90, but I'm not aware of a 3.90 gear available for the 8.8 rearend.