rear end ratios... gimme a lession

[Kojack]

i was doing some research, and I found out that thankfully, the rear end I've got in my kaiser will not blow into bits when I put a V8 behind it. I found out as well that the ratios could be kind of random too, but they all seem high... I belive one KF guy told me it could be a 4.09 or possibly a 4.55. I don't know much about ratios, other then those gears are supposed to be kinda high. Are those more for acceleration? Will I be taching high with freeway crusing? The first number has to do acceleration I think... what's the second?

thanks for any help.

[Phil1934]

The 4.55 will give you about 3400 RPM at 60 MPH with a 27" tire w/o overdrive and the 4.09 is 3050 RPM. I've had seveeral 4.10 geared vehicles and depending on insulation, you can live with it. (I'm not big on car radios.) A 3.55 is a good all around ratio or 3.73 for city and 3.00 for a lot of freeway driving.

[Kinky6]

Hey Kojack, If you have a rear end ratio of 4.09:1 or 4.55:1 (four.five five to one), this means that for every gear tooth on your pinion (input from trans) gear, you have 4.55 teeth on your ring (output to tires) gear. This mathematical relationship determines the amount of torque multiplication that your rear end has. A larger number, like 4.55:1, is referred to as a low gear ratio, while a smaller number, like 3.08:1, is called a high gear ratio.

A low gear ratio will get you moving faster from a standstill, because it will allow your vehicle to overcome its inertia, but it will have your old Kaiser engine near redline at 70-75 mph. A high gear ratio will not yeild as much torque multiplication for getting started, but will allow a lower engine speed in freeway traffic.

All of this assumes a non-overdrive 3 or 4 speed trans, where the top gear is a 1:1 output. Overdrives allow you to take advantage of these ratios by using, say, a 4.09:1 to get you moving in traffic, and then when you reach cruising speed in traffic, shifting into an overdrive gear like 0.75:1, which will effectively lower your rear end ratio to about 3.08:1, and allowing lower engine rpm's.

Don't let me confuse you with the difference between rear end ratios and transmission ratios, but they both affect torque multiplication. Also, remember that a taller tire, or one with a larger diameter, will effectively give you a higher rear end ratio than a shorter tire. These aren't the best links that I've found on this, but try -

http://www.xse.com/leres/ss/calculator.html

http://www.corral.net/tech/gearcalc.html

Hope this helps, Kinky6 http://www.jalopyjournal.com/ubbthreads/images/graemlins/cool.gif

[Missing Link]

Excellent 'splaination Kinky!

[CruZer]

Link nailed it!!! Now even I understand better.
Kojak,your car probably does have 4.xx to 1 rear gears. Most 30-50's cars with 6 cylinders and standard trans had low gears in the rear because there wasn't much horsepower or torque in those motors to get a car moving off the line and there weren't many freeways where anyone could run much over 45 -50 mph.

[Deyomatic]

One more thing, Kojack, just for reference, THIS is sure to confuse you...
You kept saying that your gears were HIGH, they aren't. They are LOW. The higher the first number is the lower the gearing. Think about it this way, in your tranny, for first gear (LOW gear) you want about a 3:1 ratio, so that you have the torque to get this 2-ton beast moving from a standstill. When you get up to speed, you shift into a HIGHer gear, 2nd or 3rd or 4th, If you DON'T have Overdrive, your HIGHest gear is going to have the lowest number, 1:1.
Same for rear ends, when it is geared really low, it has a higher number:1 to get the car moving quicker, for accleration. Like for drag racing. When you have a lower number:1 it will seemingly take forever to get the car moving if it is heavy, but once it is moving, you can go 150 MPH.
I had a car with a 2.73:1 rear axle. 350 HP engine. The fastest I could ever run this at the strip was 16.2@ at around 87.xx MPH. Now, when I got it onto the highway leaving there, I could theoretically go 150 MPH and only be revving at about 5000 RPMs. Then I could shift it into OD (EVEN HIGHER) and keep the RPMs at 3500ish at 150 MPH.

[Missing Link]

One more thing that may help you understand a bit more...or maybe not.
Take for example a 3:1 ratio ~ this basically means that one gear is turning 3 revolutions to the mating gears 1 revolution.
A set up with 2.73:1 gears would get you 2.73 revolutions of one gear for every 1 revolution on the mating gear.

[Deyomatic]

Or, to further Link's explanation, the DRIVESHAFT would be turning 2.73 times to the Rear axles' 1.

[Kojack]

Okay, cool... this is quickly making more sense. Before I didn't know that the hell the number even ment, so I'm getting pretty far here. For me, at least. The toughest part of it is realizing the numbers work backward... a 4.55 is a lower ratio as opposed to a 3.08, which is higher. A low gear ratio is also called a tall ratio as well?

Deyo cleared it up my last question, which was what each number related to on the car. Makes a lot of sense, actually... thanks a lot, guys. I'm always up to learning something new.

So, I suppose this presents a new issue then... I've got a high winding rear end, that when I'll be crusing on the freeway is going to possibly be taching me out quick. Gonna be nice for around town though. I suppose I'll have to bolt all this shit up and see what happens. There is a guy around town who's got a 305 Chevy motor that as built up in a Speed Shop in Napa I think I'll be getting. Should make that care a quick bugger.

[Deyomatic]

Now that you get that part, chew on this. Your tire size also makes a big difference, but only when you are rolling. The shorter the diameter of the tire, the higher it will rev. It gets really confusing when you think about this part: When your wheels are up in the air, and you have a 3.00:1 axle, the driveshaft will spin 3 times per wheel revolution, no matter what size the wheel is. It is when you put it on the ground and roll it that the tire diameter matters, because of circumference and distance. It still spins the same ratio, but the distance is different.

So, the next time you see some asshole in a 92 Accord with 12" rims, you can laugh at his dumb ass for taking thousands of miles off the life of his engine by revving it so much higher than it needs to be reved. At the same time, when you are sitting at a light in your Escalade with stock diameter rims, and the gangsta pulls up next to you on his DUBS, you'd out accelerate him, assuming wheel weight wasn't a factor, because your gear would be revving higher.


If you found a 700R4 or 2004R to use with the 305, you'd love those 4.xx gears. Best of both worlds. They are a bit pricier than your other autos though. Don't get one made before 1987 or so, I think.

[Kojack]

Interesting... I didn't realize HOW much tire size relates to engine revs and engine wear. That really makes that dipshit around town with a '94 Saturn and 12 inch gold wire wheels seem even dumber.

I'm running 15's on my Kaiser... the stock rims. I like how they fill the wheelwell, and the best caps I like seem to be all for 15's. What you recogmend for tire sizes on the rear? Should the match the front as well? I suppose as long as it's a fairly resonable size, I should be okay.

My only question that remains is about how transmissions change the rear end ratios when they change gears... but that I'll hit the libary about, as it's really got me interested in this. Most of the car guys I hang around are about my age, and know ones really been able to answer what the hell ratios are. Now I know. http://www.jalopyjournal.com/ubbthreads/images/graemlins/smile.gif

[Hellfish]

this has been really helpful!
I'm going to hijack kojack's post for a sec. i've been trying to find 3.5 or 3.7 gear rearend for my 48 which will have a 235. The real advantage to the 235 is it's torque and ability to get off the line, but i'm going to be putting a lot of highway miles on this car, so will I be shooting myself in the foot with 3.5 gears, i.e., losing the torque advantage? Would I be better off with 4.11 gears and a T5 or similar tranny with OD? I was going to use the 3 speed tranny from a 60 Chevy.

[Deyomatic]

I think you add them, but I'm not really sure. Like if your Axle is 4.0:1 and your first gear is 3.0:1, I think you come up with 7.00:1, then you figure in your tire size.

You might want to play around on the Richmond site, for axle ratios.
http://www.richmondgear.com/101032.html

[Deuce Roadster]

http://www.jalopyjournal.com/ubbthreads/images/graemlins/grin.gif


You multiply them..............a 4.0 rear ratio with a 3.0 first gear would be 12 to 1 http://www.jalopyjournal.com/ubbthreads/images/graemlins/grin.gif




http://www.jalopyjournal.com/ubbthreads/images/graemlins/crazy.gif

[Deyomatic]

Yeah, Deuce, I just figured that out, I was having a hell of a time pictureing it until I drew up a diagram.

if your engine is spinning at 2400 RPMs, in first gear (3:1) your driveshaft is spinning 800, then your wheels (4:1) are spinning 200.

2400 divided by 200 = 12. So you multiply the ratios.



I shouldn't forget again.

[Kinky6]

Kojack - I just gotta ask, did you shave your head today? http://www.jalopyjournal.com/ubbthreads/images/graemlins/grin.gif

O.K., for what its worth, a tall rear gear is another way of saying a high gear ratio.

Hellfish - you're probably looking at pretty much the same thing I was with your '48. I wanted to get a 3.73:1 S-10 rear axle for my '39 Chevy w/ 302" GMC six, good width, would work well with my '86 Camaro T5 trans. All I could find around here were 3.40:1 or 4.11:1 rears, so I took the four-eleven. With the 5th gear in the T5, that would give me an effective 3.08:1 with my 25" tall rear tires.

But, with the 2.95 1st gear x that 4.11:1 rear, I would wind out my 12.12:1 gear in about the time it took to let the clutch out. I've heard opinions that an effective starting ratio of about 9.0:1, up to maybe 10.0:1, is a good target for a standing start. Plus, I still wanted a little lower rpm's at 70mph. My solution is to go from a 25" tall tire to a 30" tire on a 16" Torq Thrust copy-cat rim.

This change in tire size will make my 4.11:1 rear act like the 3.73:1 that I wanted, with about a 2.79:1 effective ratio in overdrive, and around 2550 or 2600 rpm @ 70mph. This will also get me a more usable 11.00:1 1st gear, and, most importantly, give me a really bitchin' rubber rake, 'specially with some 14" fronts.

More links:

www.SMOKEmUP.com (http://www.SMOKEmUP.com)

Click on the Auto Math tab, lots of good stuff in there.

http://50chevy.freeservers.com/suspension_widths.html

Is another good link.

http://inliners.org/

Go to the Tech Tips page, then to the T5 Tranny page, more good stuff.

Whew! http://www.jalopyjournal.com/ubbthreads/images/graemlins/tongue.gif



Later, Kinky6 http://www.jalopyjournal.com/ubbthreads/images/graemlins/cool.gif