Differential gear questions!!!
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Differential gear questions!!!
Hi everybody! My 1980 will be rolling any time now. I've got the crate engine turned (btwy is there anything worse than second-guessing yourself about how much HP you chose? Seems like opinions and rare barn-find super-fast engines are crawling out of the woodwork now that my 350 zz4 is bolted in.) and with some minor clean up I'll be on the road.
My next project will be figuring out what to do with the rear end gears. The last time I drove the car, I was revving at 5000 going 55-60. That's not what I want. I want to have a respectable departure from the traffic light, but I'm not trying to race my car. I want an about-town driver and a car I can take on the highway for short road trips.
Unfortunately I can't tell you all what I have in the car in the way of transmission and gearing at the moment. The guy I bought it from disappeared when I handed him the check, and he did a lot of haphazard changes all over. I've heard I might have 4.11's, but I don't know. What I'm looking for is general information on gearing.
I've read a lot of posts about gearing, but most of these posts assume a level of knowledge I don't have. Let me ask these questions:
1. What is the practical reason for choosing a gear ratio that makes your drive shaft spin more for fewer tire rotations (a low gear ratio). Does this have something to do with torque?
2. Where does the number for a gear ratio like 4.11 come from? Is this comparing a certain gear in the tranny to the rear gear?
3. I've asked a number of people which gear ratio I should put in my car. The consensus seems to be that 3.08's will by doggish around town and that 3.55's would be great. Why are 3.08's "doggish"? What would make 3.55's better?
Any guidance would be appreciated.
Thx
frankjuka
My next project will be figuring out what to do with the rear end gears. The last time I drove the car, I was revving at 5000 going 55-60. That's not what I want. I want to have a respectable departure from the traffic light, but I'm not trying to race my car. I want an about-town driver and a car I can take on the highway for short road trips.
Unfortunately I can't tell you all what I have in the car in the way of transmission and gearing at the moment. The guy I bought it from disappeared when I handed him the check, and he did a lot of haphazard changes all over. I've heard I might have 4.11's, but I don't know. What I'm looking for is general information on gearing.
I've read a lot of posts about gearing, but most of these posts assume a level of knowledge I don't have. Let me ask these questions:
1. What is the practical reason for choosing a gear ratio that makes your drive shaft spin more for fewer tire rotations (a low gear ratio). Does this have something to do with torque?
2. Where does the number for a gear ratio like 4.11 come from? Is this comparing a certain gear in the tranny to the rear gear?
3. I've asked a number of people which gear ratio I should put in my car. The consensus seems to be that 3.08's will by doggish around town and that 3.55's would be great. Why are 3.08's "doggish"? What would make 3.55's better?
Any guidance would be appreciated.
Thx
frankjuka
Safety Car
Joined: Oct 2006
Posts: 3,993
Likes: 9
From: Gig Harbor WA
Hi everybody! My 1980 will be rolling any time now. I've got the crate engine turned (btwy is there anything worse than second-guessing yourself about how much HP you chose? Seems like opinions and rare barn-find super-fast engines are crawling out of the woodwork now that my 350 zz4 is bolted in.) and with some minor clean up I'll be on the road.
My next project will be figuring out what to do with the rear end gears. The last time I drove the car, I was revving at 5000 going 55-60. That's not what I want. I want to have a respectable departure from the traffic light, but I'm not trying to race my car. I want an about-town driver and a car I can take on the highway for short road trips.
Unfortunately I can't tell you all what I have in the car in the way of transmission and gearing at the moment. The guy I bought it from disappeared when I handed him the check, and he did a lot of haphazard changes all over. I've heard I might have 4.11's, but I don't know. What I'm looking for is general information on gearing.
I've read a lot of posts about gearing, but most of these posts assume a level of knowledge I don't have. Let me ask these questions:
1. What is the practical reason for choosing a gear ratio that makes your drive shaft spin more for fewer tire rotations (a low gear ratio). Does this have something to do with torque?
2. Where does the number for a gear ratio like 4.11 come from? Is this comparing a certain gear in the tranny to the rear gear?
3. I've asked a number of people which gear ratio I should put in my car. The consensus seems to be that 3.08's will by doggish around town and that 3.55's would be great. Why are 3.08's "doggish"? What would make 3.55's better?
Any guidance would be appreciated.
Thx
frankjuka
My next project will be figuring out what to do with the rear end gears. The last time I drove the car, I was revving at 5000 going 55-60. That's not what I want. I want to have a respectable departure from the traffic light, but I'm not trying to race my car. I want an about-town driver and a car I can take on the highway for short road trips.
Unfortunately I can't tell you all what I have in the car in the way of transmission and gearing at the moment. The guy I bought it from disappeared when I handed him the check, and he did a lot of haphazard changes all over. I've heard I might have 4.11's, but I don't know. What I'm looking for is general information on gearing.
I've read a lot of posts about gearing, but most of these posts assume a level of knowledge I don't have. Let me ask these questions:
1. What is the practical reason for choosing a gear ratio that makes your drive shaft spin more for fewer tire rotations (a low gear ratio). Does this have something to do with torque?
2. Where does the number for a gear ratio like 4.11 come from? Is this comparing a certain gear in the tranny to the rear gear?
3. I've asked a number of people which gear ratio I should put in my car. The consensus seems to be that 3.08's will by doggish around town and that 3.55's would be great. Why are 3.08's "doggish"? What would make 3.55's better?
Any guidance would be appreciated.
Thx
frankjuka
Your ratio is number of driveshaft rotations per tire rotation. 4.11's have the driveshaft (indirectly the motor) turning 4.11 times for every turn of the tire.
3.08's likewise..so with 4.11's (higher # but "lower" gearing) your motor rpms will be higher at the same speed than say 3.08's.
For instance, at 60 mph with 4.11's, a stock TH400, and 255-60-15's, you're turning about 3000 rpms. With 3.08's you're only turning 2300 rpms, huge decrease.
This from the nifty little calculator on Bowties website..
http://www.700r4.com/speedoCalc/rpmcalc.shtml
The lower rpms will be quieter, MUCH more fuel efficient, probably less wear and tear on your motor, but will put you below the optimum power and torque bands for your engine (doggish?)
The 4.11's will give you a nice hole shot, but you'll be sucking alot more gas throughout, especially at cruise speeds.
Thats the basics...the experts on here will chime in with more specifics...
BTW, if you were doing 5000 rpms at 60, you were in 2nd or 3rd gear!!! Jack up your car and turn the driveshaft and see how many times your wheel turns, that will narrow down your ratio, also its stamped on the bottom of the differential.
Hope this helps.
