Is there a way I can check what diff ratio I have?
 

My race car is an '84, and I was underneath it today and noticed that the diff has definitely been replaced before I bought the car. I don't know for sure what ratio it has...3.90, 4.10, 4.44 ??? Is there a way I can find out?

Carefully make marks on the wheel and the driveshaft with a marker or something (or just make mental notes of where a distinctive feature is on each part).

Now while you're under the car with the wheels jacked up in the air, put it in neutral and slowly turn the driveshaft with your hand and count the rotations of the driveshaft and the rotations of the wheel. If you do enough rotations, you should hopefully be able to tell what your ratio is.

Do as wargasm said, but turn the shaft at least 10 times to make sure you are accurate.

You can also just drive the car at a constant speed in 4th gear (1:1 ratio). Check your indicated speed and revs. You can back out the final ratio from that.

If the tach and speedo are right. My Miata's tach reads a few hundred RPM high. According to the method above I was convinced I had the kinda rare 4.30 gear. Turns out it the run of the mill 4.10 which was varified by marking the driveshaft and spinning it by hand.

Yeah, definately a good point (you also have to know the correct OD of your tires). But usually the tach and speedo are close enough to figure it out.