What a great (old) post, I missed it.
Everyone pretty much nailed it on ratios, spot on..
Now, for my 2 cents. Its a system, the car, every part of it. You can look at individual parts of the system, but try not to get too focused on the individual parts, they work TOGETHER as an entire system. An understanding of the workings of each part of the system is needed.
Then, once you get the idea of how each component does its job the difficult part is to incorporate each of these sub components into the entire system, the car as a whole.
And thats the tuff part. All the auto manufactures pay thier engineers pretty good money to make the components work together as a compleate system, the car on the show room floor. Its a sacrifice, longevity and performance with a low price tag. I dont want that job!!!
It amazes me when guys do get it right. And those are the ones that are winning races (if its a racing car) or the ones that have a dependable and usable cruiser (steet car).
So now that the rear end gear ratio is understood. And how it interacts with the transmission gearing now what??
And Ill just talk about a street car cause really. The racing cars are just too varied in ALL the possibilities. And each car should be looked at individually, you cant say what works here on this car will work on another. Racing cars are completely independent of each other racing car, thats one of the reasons we have winners and losers. They are as independent as the driver sitting in the car next to them.
But street cars have more commonalities. You can pull up to a similar year and make car on the road and unless its a racing car on the street it still has alot of similar aspects of your car. Sure, it may have independent suspension, big block, fuel injected, yadda ya. But if its truly a street car, able to travel a few hundred miles up the road and back without AAA help then its still held within the limitations of a street car.
So with that..... Final rear end gear ratios are IMO the last thing that should be looked at. Why? Well, its one of the easiest of the drive train components to change.
Just my thoughts but there are several things to consider before picking the rear end gears, for a street car.
You cant change the weight of your car that much, so final weight of the car is a biggy. Horse power and torque of the engine. If you have a stump pulling BB that loves to give the torque down low then you can get away with a shorter gear, and you should use a shorter gear. Otherwise you are wasting that torque monster on a tall gear that wont push the engine. It never gets to come even close to its torque curve, you wont ever get there.
But on a nippy mouse engine, that doesnt have any torque down low but loves to show its muscle up a lil higher in the RPM band then a tall gear will help. An engine lacking on torque will love a tall gear. The tall (fast) gear allows the small mill to get into its higher RPM band to give some serious performance.
So now what, I have a mighty mouse and tall gears but Im running at 3000 rpm on the freeway just to keep up with the 65MPH crowd.. Well thats where the transmission is supposed to pick up the slack. Give the mouse a break once she is up to speed.
And so GM and all the others came up with the OD transmissions, auto and manual. Kick down the rpms alil at highway speed, where the car is already in motion, doesnt take much to push the block through the air once she is moving, toss in an over driven gear ratio. Yea!! Problem solved. Well kinda. They are a problem with carburators, the carb would go into idle if the driver slowed down to 55mph and they lost power and had to give more pedal. Tranny kicked down and brought the Rs back up for the carb to drink. Back and forth, wasnt good.
Then EFI was a perfect match, sorta. The EFI could cruise at 1800 rpm on the highway, a slight dip in on the throttle to keep a constant speed wasnt an issue, it would adjust timing and fuel. But we kinda lost control. Sensitive control. We were held back by the computer. Give it a slight tip in on the throttle and it would just kinda bog, trying to stay in the top gear. So we tipped in further and further till it down shifted. And on some cars its abrupt.
Make it worse, now we have drive by wire. I hate that. Im always fighting the throttle (computer) for where I want the power to be applied. Ummm?? I HATE my drive by wire cars I have, they drive me nutts.
OK... I digress........... Weight of the car, torque of the engine, size of the rear drive tires, transmission ratios and really, the needs of the car will determine what your rear end gear ratio will be. A BB with 600lbs of torque in a 1000lb car can live with a 2.73:1 rear, it will smoke the tires, no matter how wide. A 4000lb car with the same engine might like to have a lil taller gear to smoke the same tires
JR