Charger Forums banner

1 - 20 of 35 Posts

·
Registered
Joined
·
96 Posts
Discussion Starter #1

·
Registered
Joined
·
2,732 Posts
Stonebreaker said:
Just wondering because at http://www-5.dodge.com/vehsuite/VehicleCompare.jsp they list the rear axle ratio as 2.82 but at the bottom of the page under the powertrain tab they list the final drive ratio as 3.55 for the hemi.
the rear axle ratio is just that...

final drive ratio is the ratio of everything, including the tranny, rear end, and tires....
 

·
Registered
Joined
·
2,425 Posts
If you are talking about the lx hemi cars, the axel is 2.82 and 5th gear is .82 overdrive.
Then the final drive ratio is 2.31 (2.82x.82)
I did not see the information you referred to.
________
ExoticaSQUIRT live
 

·
Premium Member
Joined
·
15,199 Posts
Except of course, the SRT:)
 

·
Registered
Joined
·
96 Posts
Discussion Starter #5
DAYTONA_R/T said:
the rear axle ratio is just that...

final drive ratio is the ratio of everything, including the tranny, rear end, and tires....
What do the tires and the tranny have to do with the final drive ratio?
 

·
Registered
Joined
·
2,425 Posts
Yes the SRT has a 3.06 rear end ratio and with the .82 overdrive that makes for a 2.51 final ratio.
The SE and SXT have 2.87 rear end ratio and the same .82 overdrive making the final drive ratio of 2.35.
I don't think tire size is typically included in calculating the final drive ratio. They are not part of the drive train.
________
LIVE SEX
 

·
Registered
Joined
·
352 Posts
maneval is rite .a 2.82 ratio is just that. with a 24 in tall tire or a 30 in tall tire and with a 4th gear tranny ratio of 1-1. its just they turn diff rpm at the same speeds. a car with 2.82 rearend gears with a 24 in tall tire at 60 mph will tack 2450 and with a 30 in tall tire it would tack 1875. the overdrive ratio is 0.83 not 0.82 but thats close enought. now 3rd gear ratio is 1.41 -1 so with a 2.82 rear end ratio the overall 3rd gear would be 3.97-1. tire size only changes the rpm not the ratio.
 

·
Registered
Joined
·
1,855 Posts
You're going the wrong way... you have to divide the final trans ratio .83 by the axle ratio to get the "final drive ratio". since the final gear(5th) is overdriven. in 4th the trans is at 1 to 1 ratio, meaning that every revolution of the drive wheel results from 2.82(axle ratio) turns of the driveshaft. When the transmission is over-driven, it turns a percentage of a turn ( in the case of the charger .83 percent) to move the driveshaft. when the drive wheel completes 1 revolution on over drive the drive shaft has moved 3.4 revolutions, thus the "final drive ratio" 3.40 driveshaft revolutions to 1 axle shaft revolution. in third gear ( 1.41 to 1) you divide the axle ratio by the gear ratio and you get 2, meaning that every rotation of the drive wheel only requires 2 revolutions of the drive shaft, 2nd gear (2.19 to 1) you get 1.29 revolutions of the drive shaft to move the drive wheel 1 revolution and first (3.58 to 1) requires only 78% of a turn of the drive shaft to move the drive wheel 1 revolution! whew! perhaps DCx has figured the drive lash into the equation to come up with 3.55, or maybe it was a mistake ( like the 375 rating on torque
 

·
Registered
Joined
·
2,732 Posts
maneval69 said:
I don't think tire size is typically included in calculating the final drive ratio. They are not part of the drive train.
coming from the "truck world" yes, tire size affects final drive ratio... on cars you normaly dont drastically change the overall diameter of the tires that this would effect any changes you have made to a vehicle...

Changing tire size affects the final drive ratio. Switching from a 30" tire to a 35" tire changes the final drive ratio by about 17%. This may drop the engine out of its' "power band" and result in poor performance and fuel economy. To restore performance you must change the gear ratio to compensate for the change in tire size. If you originally had 3.07 gears you need a ratio that is approximately 17% lower, such as 3.55. If you want to increase off road performance you might want a 4.10 or lower ratio.
taken from

http://www.differentials.com/faq.html


Jim, you said that changing tire size changes the RPM.... well if you change any gear you change the RPM of the engine... and changing the tire to a different size will change the final drive ratio, and RPM.... the two go hand in hand....

As said above... tires is something most car people dont think about when it comes to gearing... as you dont vary too far from stock tire size on cars....
 

·
Registered
Joined
·
2,732 Posts
Final Drive Ratio: The final drive ratio will reflect how many times the countershaft will rotate for every rotation of the rear wheel. The greater this number, the lower the gear. As the gear gets lower, ability to accelerate goes up and peak speed goes down.
taken from

http://www.peterverdonedesigns.com/fdrselection.htm

Now keep in mind, a change in tire size will change the "rotation of the rear wheel" which in turn will change the final drive ratio

if you want to see the revolutions per mile of your tires... you can use this like http://www.miata.net/garage/tirecalc.html (ignore the miata thing... tire sizes are avalible for cars and trucks on it...)
 

·
Banned
Joined
·
27 Posts

·
Registered
Joined
·
2,425 Posts
Final drive ratio is how many rotations the AXLE makes compared to the crankshaft.
Tire size doesn?t matter. If you have 25 ? tire or a 30? tire, the tire will rotate at the same rpm as the axle. (if you are calculating performance the tire size is always a different entry than final drive ratio)


Chargershed,
You have it backwards. The ratio is always (input:eek:utput) Edited # 2. The rear gear is 2.82:1, the transmission in 5th gear is .83:1. Overdrive spins the output shaft of the transmission (driveshaft) faster than the input shaft of the transmission (crank) or engine rpm. The .83 ratio means that for every 1 rotation of the output shaft (drive shaft) the input shaft (crank) spins .83 times.
So running a 2.82 rear gear in a .83 overdrive gear would give you the same final drive ratio as running a 2.34 rear gear and our trans in 4th (which is 1:1 gear)

I may have missed the boat on what final drive ratio is referring to but I?ve always referred to it as the ratio between the crank and the axle = drive-train. And you have a final drive ratio in every gear.
Here are our transmission gear ratios; 1st =3.59 2nd =2.19 3rd = 1.41 4th = 1-1 5th =0.83.
You multiply those by the rear gear 2.82 to get each gears final drive ratio.
And here are the r/t?s final drive ratios for each gear.
1st -10.12
2nd -6.18
3rd -3.98
4th -2.82
5th -2.34
________
Glass pipe
 

·
Registered
Joined
·
2,425 Posts
Now keep in mind, a change in tire size will change the "rotation of the rear wheel" which in turn will change the final drive ratio
wrong! tire size will effect the distance traviled by one rotation it DOES NOT effect the ratio that the tire turns in relation to the crank therefore it does not affect final drive ratio.

In 5th gear for every rotation of the tire you have 2.34 rotations of the engine. It doesn?t matter what size the tire is. Yes, tire size effects how speed relates to rpm and performance but tire diameter is always considered independent of final drive ratio.
________
Sunny live
 

·
Registered
Joined
·
352 Posts
daytona thats true but you still have a 2.82 rearend gear no mater what tire size you use that what i was getting at.if you have a 24 in tire and you put a 30 in tire on yes your overall ratio will change to a 2.22 but you will still will have a 2.82 rearend gear. now this is all done with a tranny gear that is 1-1.
 

·
Registered
Joined
·
1,855 Posts
maneval69 said:
Final drive ratio is how many rotations the AXLE makes compared to the crankshaft.
Tire size doesn’t matter. If you have 25 “ tire or a 30” tire, the tire will rotate at the same rpm as the axle. (if you are calculating performance the tire size is always a different entry than final drive ratio)


Chargershed,
You have it backwards. The ratio is always (input):(output). The rear gear is 2.82:1, the transmission in 5th gear is .83:1. Overdrive spins the output shaft faster than the input shaft or engine rpm. The .83 ratio means that for every 1 rotation of the output shaft (drive shaft) the input shaft (crank) spins .83 times.
So running a 2.82 rear gear in a .83 overdrive gear would give you the same final drive ratio as running a 2.34 rear gear and our trans in 4th (which is 1:1 gear)

I may have missed the boat on what final drive ratio is referring to but I’ve always referred to it as the ratio between the crank and the axle = drive-train. And you have a final drive ratio in every gear.
Here are our transmission gear ratios; 1st =3.59 2nd =2.19 3rd = 1.41 4th = 1-1 5th =0.83.
You multiply those by the rear gear 2.82 to get each gears final drive ratio.
And here are the r/t’s final drive ratios for each gear.
1st -10.12
2nd -6.18
3rd -3.98
4th -2.82
5th -2.34
But, using your math, means the engine crank spins 10 times for every one turn of the output shaft- the very definition of "overdriven" - in first gear! wouldn't the reverse be true? I'm so confused!! It would seem that there are many diffrent definitions to "final drive ratio".
 

·
Registered
Joined
·
2,425 Posts
Good catch,
I miss-typed it should be (output:input).
In first gear the crank spins 10.12 times for every 1 rotation of the tire.
I have edited my earlier post.maybe it make more sense now.
Thanks
________
GONG BONG
 

·
Registered
Joined
·
2,732 Posts
maneval69,

I posted a link with info to support my reasoning that tires play a factor in final drive ratio, in post #9 on page 1 of this thread... can you post information otherwise stating that this information is wrong?

patiently waiting for proof other then "cause I said so" :)
 

·
Registered
Joined
·
630 Posts
maneval69 said:
Good catch,
I miss-typed it should be (output:input).
In first gear the crank spins 10.12 times for every 1 rotation of the tire.
I have edited my earlier post.maybe it make more sense now.
Thanks
Damn, I didnt want to get in the middle of all this, but maneval69, I think you were right in the first place, (input : output). In 1st gear, the crank spins 10.12 times per 1 revolution of the axel. Thats why you are only doing a slow speed at 5500rpm in 1st gear (rear axel is spinning at 592.88 revs). Switch to 2nd gear and you can go faster at the same RPM because the final drive is now 6.18 cranks to 1 axle rev (@5500/6.18 = 898.69 axle revs). Same for the other gears, 3rd (3.98) gets 1381.91 axle revs, 4th (2.82) gets 1950.35 axle revs, and 5th (2.34) gets 2350.43 axle revs.

Of course this is all theoretical, the actual speed @5500 in 5th the variables of drag, air density, real HP, TQ slip, etc come into play as to how fast the car would be going.
 

·
Registered
Joined
·
2,425 Posts
from your link
Changing tire size affects the final drive ratio. Switching from a 30" tire to a 35" tire changes the final drive ratio by about 17%. This may drop the engine out of its' "power band" and result in poor performance and fuel economy. To restore performance you must change the gear ratio to compensate for the change in tire size. If you originally had 3.07 gears you need a ratio that is approximately 17% lower, such as 3.55. If you want to increase off road performance you might want a 4.10 or lower ratio.
They do agree with you as to the meaning of final drive ratio.
here is another that agrees with you
But this statement is not correct or at best not complete.
Now keep in mind, a change in tire size will change the "rotation of the rear wheel" which in turn will change the final drive ratio
From these definitions of final drive ratio, the wheel diameter will change the rotation of the wheel as it relates to speed of the car but does not change the wheels rotation in relation to the crank. An important point.

So here forth I shall include tire diameter as part of the final drive ratio.

Now can any body answer the the original question
Just wondering because at http://www-5.dodge.com/vehsuite/VehicleCompare.jsp they list the rear axle ratio as 2.82 but at the bottom of the page under the powertrain tab they list the final drive ratio as 3.55 for the hemi
________
WEB SHOWS
 

·
Registered
Joined
·
2,425 Posts
CO-Charger said:
Damn, I didnt want to get in the middle of all this, but maneval69, I think you were right in the first place, (input : output). In 1st gear, the crank spins 10.12 times per 1 revolution of the axel. Thats why you are only doing a slow speed at 5500rpm in 1st gear (rear axel is spinning at 592.88 revs). Switch to 2nd gear and you can go faster at the same RPM because the final drive is now 6.18 cranks to 1 axle rev (@5500/6.18 = 898.69 axle revs). Same for the other gears, 3rd (3.98) gets 1381.91 axle revs, 4th (2.82) gets 1950.35 axle revs, and 5th (2.34) gets 2350.43 axle revs.

Of course this is all theoretical, the actual speed @5500 in 5th the variables of drag, air density, real HP, TQ slip, etc come into play as to how fast the car would be going.
Damn-it your right!
Talked me right into it.
I got to stop trying to understand math while I'm drinking.
I will edit it again!!
________
Lax Patient Remodies
 
1 - 20 of 35 Posts
Top