Gear ratio

BL1 70 roadrunner

BL1 70 roadrunner

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Hey guys I'm trying to figure out what gear ratio means. I've looked up online what they mean but all I've gotten out of it is that 4.10 is best for drag racing 3.23 is best for highway and normal use. I'm wondering if anyone can sorta clear this up for me. Our roadrunner has a 3.91 ratio is that good or bad? Thanks for the help.
 
1:3.91 means the driveshaft needs to turn 3.91 times for 1 full turn of axle/wheel.

3.91 is nice "in between"gear if you ask me.
3.23/3.55 are more like highway gears.
if you go 4.10 or more is race only/ in town fun.

But then again i live in the Netherlands.
Maybe teh usa drives have a different thought.
 
Example 3.91 will turn more RPM at freeway speed 60MPH than 3.23 will. Depending on your goals and driving style Street 3.23 or Race 4.10. I would say 3.91 is a good compromise for both street and race.
 
BL1 70 roadrunner said:
Hey guys I'm trying to figure out what gear ratio means. I've looked up online what they mean but all I've gotten out of it is that 4.10 is best for drag racing 3.23 is best for highway and normal use. I'm wondering if anyone can sorta clear this up for me. Our roadrunner has a 3.91 ratio is that good or bad? Thanks for the help.
Click to expand...

Does your RR have an A31 on the fender tag?

If so it came with the High Performance Axle Package w/3.91 Ratio.
 
Yep 3.91 Sure Grip, 440/Hemi leafs, torsion bars, and sway bar, and skid plate, 26" fan and shroud and clutch fan, power steering cooler (if PS).

My 70 has this option too. I love the Heavy duty suspension.
 
The A31 package was only available on a 70 RR with 383 but you could probably have got the 3.91s separately or they could have easily been swapped in by a previous owner during the cars existence.
 
BL1 70 roadrunner said:
So does the higher gear ratio need more RPMS?
Click to expand...
There is a little mistake with the semantics in the above responses. The higher the rear ratio number, the lower the gear ratio. In other words, a 4.10 ratio is lower than a 3.23 ratio. A low gear requires more rpm to go the same speed as a higher gear. In the same way that the first gear in the 4 speed transmission is considered low gear but it has a 2.65:1 ratio as compared to high gear, which is a 1.00:1 ratio. The high gear takes less rpm to go faster. For drag racing, the hole shot is affected by the overall gear multiple in first gear. A 3.91 x 2.65 has a 10.362 multiple vs 3.23 x 2.65 or an 8.56 multiple. The lower 3.91 rear has more than a 20% higher multiplication factor than a 3.23 which gets the tires turning faster and the rpms up into the peak torque range a lot faster than a 3.23 ratio. The T/A Challengers and AAR Cudas came stock with 3.91"s. It's a great rear ratio for overall performance.
 
tallhair said:
The A31 package was only available on a 70 RR with 383 but you could probably have got the 3.91s separately or they could have easily been swapped in by a previous owner during the cars existence.
Click to expand...

Not so. The A31 performance axle package was available in 1969 on 383, 440 and I believe Hemi automatic cars.
 
RoadRunnerJD said:
Not so. The A31 performance axle package was available in 1969 on 383, 440 and I believe Hemi automatic cars.
Click to expand...

Sorry I wasn't clear. I was answering what the A31 package was available on specific to the 70 RR since his car is a 70 RR and he asked what engines the 3.91 ratio was available on. Granted he doesn't have that package and I didn't go do any research to see what the optional gear ratios were outside of a package. I'm familiar with the A31 because my 70 RR has it and 68-70 B bodies options generally.

Yes the A31 package was available on other years and on at least 70 E bodies but I don't keep up with those or A and other bodies option availability

quick search found this but it may not be all inclusive for optional gear ratios outside of the performance axle packages.

from hamtramck .. salemans' pocket guide http://www.hamtramck-historical.com/images/dealerships/SalesmansPocketGuides/1970/Pg 017.jpg

optional 70 gear ratios in b bodies.jpg

optional 70 gear ratios in b bodies 2.jpg

optional 70 gear ratios in b bodies 3.jpg

optional 70 gear ratios in b bodies.jpg

optional 70 gear ratios in b bodies 2.jpg

optional 70 gear ratios in b bodies 3.jpg
 
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tallhair said:
Sorry I wasn't clear. I was answering what the A31 package was available on specific to the 70 RR since his car is a 70 RR and he asked what engines the 3.91 ratio was available on. Granted he doesn't have that package and I didn't go do any research to see what the optional gear ratios were outside of a package. I'm familiar with the A31 because my 70 RR has it and 68-70 B bodies options generally.

Yes the A31 package was available on other years and on at least 70 E bodies but I don't keep up with those or A and other bodies option availability

quick search found this but it may not be all inclusive for optional gear ratios outside of the performance axle packages.

from hamtramck .. salemans' pocket guide http://www.hamtramck-historical.com/images/dealerships/SalesmansPocketGuides/1970/Pg 017.jpg

View attachment 8315

View attachment 8316

View attachment 8317
Click to expand...

Good info! Looks like a 3.91 was not available for the GTX! No wonder my 383 with the 4.30 rear would toast all those GTX's with 3.23's and 3.54's!
 
tallhair said:
Sorry I wasn't clear. I was answering what the A31 package was available on specific to the 70 RR since his car is a 70 RR and he asked what engines the 3.91 ratio was available on. Granted he doesn't have that package and I didn't go do any research to see what the optional gear ratios were outside of a package. I'm familiar with the A31 because my 70 RR has it and 68-70 B bodies options generally.

Yes the A31 package was available on other years and on at least 70 E bodies but I don't keep up with those or A and other bodies option availability

quick search found this but it may not be all inclusive for optional gear ratios outside of the performance axle packages.

from hamtramck .. salemans' pocket guide http://www.hamtramck-historical.com/images/dealerships/SalesmansPocketGuides/1970/Pg 017.jpg

View attachment 8315

View attachment 8316

View attachment 8317
Click to expand...

I think you could get up to a 4.56 ratio dealer installed but not through a factory package?
 
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I run a 30 inch tall M/T drag radial rear tire with a 4.10 and it isn't horrible on the highway. I wouldn't want to go very far.
 
RoadRunnerJD said:
There is a little mistake with the semantics in the above responses. The higher the rear ratio number, the lower the gear ratio. In other words, a 4.10 ratio is lower than a 3.23 ratio. A low gear requires more rpm to go the same speed as a higher gear. In the same way that the first gear in the 4 speed transmission is considered low gear but it has a 2.65:1 ratio as compared to high gear, which is a 1.00:1 ratio. The high gear takes less rpm to go faster. For drag racing, the hole shot is affected by the overall gear multiple in first gear. A 3.91 x 2.65 has a 10.362 multiple vs 3.23 x 2.65 or an 8.56 multiple. The lower 3.91 rear has more than a 20% higher multiplication factor than a 3.23 which gets the tires turning faster and the rpms up into the peak torque range a lot faster than a 3.23 ratio. The T/A Challengers and AAR Cudas came stock with 3.91"s. It's a great rear ratio for overall performance.
Click to expand...

John, that is one of the best explanations of the terms "lower gear ratio" and "lower rear end ratio" that I have read! Good job and THANKS!

MikeR
 
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