What's the difference between rear end gears and also B/BR engines?

[Mo Power]

Ok so I'm going to turn 18 in like a week and I have a tendacy to try to learn as much as possible about alot of sysytems on cars because I havent had much experience with engines or anything really yet. But these  things having been bothering me about why there so many rearends and the gear sets I sure there for depending on exactly what you need and your horses are under the hood. So this is just what seems to be is that the higher the gear ratio like 4.10 is for a high horsepower car and smaller then that would be for small blocks. And also I keep seeing these B/BR engines I know a difference between small block and big block but now I'm just confused. Any help for a kid trying get going on his passion for Mopars. Thanks

[Charger1973]

Ok I will attempt to answer this for you.  If I make a mistake im sure someone will tell me but since im still learning too maybe it will make more sense coming from me.   

4:10 gears would be really steep gears and in my opinion not good for much more than drag racing.  The lower number you go the better mileage you could get, but you sacrifice off the line power.  Thats why you refer to some gears as "highway" gears.   If you took those 4:10 gears on the drag strip you would launch fast, but on the highway you would have the RPMs rapped out in no time and get some horrible mileage. 

The engine thing isnt too hard.  You already know the SB and BB motors.  Well in big block some of the blocks are taller than others.  I beleive RB stands for raised block?  Anyway those blocks are taller.  The "B" motors are the 383, 400. The RB is the 440.  There are other motors that would go in these categories as well but these are the stock BB Charger motors anyway. 

[Ghoste]

If it helps, think of the rearend gear as a lever and the different ratios represent different lengths.  The idea is to get your engine into a specific powerband at the point where you want it.  Like levers, a short one will move far but takes more effort and time to reach it's zenith.  A long one can get to the top quickly and easily but not go as high.
Oversimplifying and not really the best example but if it helps?...

[Mo Power]

Thanks that gives me more of an understanding.

[Chatt69chgr]

There are also different kinds of rear ends used in Chrysler cars.  They are usually designated by a number like 8-3/4.  That refers to the diameter in inches of ring gear.  Each of the different rear ends can handle a certain amount of horsepower.  The 7-1/4 was a light duty used in 6 cyl applications.  The 8-3/4 was used in mid-sized and full sized cars up until around 1969.  Then they went to a 8-1/4.  The big daddy is the Dana 60.  It's a truck rearend adapted to car use.  It has a 9-3/4 inch ring gear and can handle in excess of 1000 hp.  Each of these rear end types came with various rear end ratios depending on the engine, transmission, and intended end use of the car.  Search the web and you will find lots of info on the things you ask about.  Google is your friend.  Before the internet, a lot of the info was just about impossible to find.  Now it just a click away.

[kab69440]

The 8.75 was in use long after 1969. They were even available with disc brakes in the 72-74 Imperials.

[RD]

B engines include:  350 ('59 only, I believe), 361, 383, 400

RB engines include: 383 ('59 only, I believe), 413, 426 wedge (not hemi), and 440

[Ghoste]

And technically, since the 426 Hemi was developed from the raised block it could called a close first cousin.