difference in differentials?

[PKRWUD]

The smaller numbers you are referring to is the gear ratio. It is the number of times the driveshaft rotates for one complete rotation of the driven axle. For example, a 4.10:1 gear set means that for every complete revolution the driven axle does, the driveshaft will rotate 4.1 times. The ratio is always given as a number (xx) to one (1). there are 2.73:1, 3.08:1, 3.23 :1, 3.56:1, 3.73:1, 4.10:1, etc. Each of those number 1's equals the axle rotating one time, while the number in front of it is how many times the driveshaft turned. Higher numbers are actually lower gears. The lower the gear (higher numerically), the faster the car will accelerate, to a point. You see, the more times the driveshaft must turn, then the more times the engine must turn, and a quickly turning engine is a faster engine. The problem comes when your engine reaches it's limits. If your engine can only handle a certain rpm, you need to understand that by spinning the engine faster, you will reach that limit sooner. In otherwords, a set of 4.10:1 gears will make a car very, very quick for a short distance, while a higher gear (lower numerically), like 2.73:1 gears, will be slow at first, but will not reach their rpm limit for quite a while, and by the time they do, their top speed will be almost twice that of the 4.10's (everything else being equal). Last analogy:

Did you ever own and ride a multi-speed bicycle, like a 10 speed? Same thing. In first gear, you had to spin the crank and pedals really fast to get the back tire to move, but because you were spinning so fast, you initially moved quickly. before you knew it, you had to shift because you had reached your rpm limit. If you had started out in 10th gear, it would have taken you forever to get going, but once you did, you would fly.

Really simple: Number of driveshaft revolutions (4.10) to (:) the number of driven axle revolutions (1)

The larger numbers you asked about refer to the size of the ring gear. The rear end has two gears: a pinion gear, that is directly attached to the driveshaft, and a ring gear, which is attached to the axle(s). They are in constant contact with each other. In a 4.10:1 ring and pinion set, the pinion gear has 10 teeth, while the ring gear has 41 teeth. Divide the pinion by the ring, and you come up with 4.1. The larger the ring gear, the stronger it is, and the more force it can handle. Ford's 7.5" rear end is called that because the ring gear measures 7.5" across. They suck. The Ford 8.8 rear end has a ring gear that measures, you guessed it, 8.8" across. It's a decent rear end. The Ford 9" rear end is not only stronger (with it's 9" ring gear), but is convenient too, because of it's design. That's a whole nother story, though, and my fingers are getting tired.

I hope that helped!

Take care,
-Chris