Final drive ratio

[RMH]

If any transmission guru's are out there i would apreciate your help. I

have a auto 3:55's. I run the truck with the od off when 55-60mph (2000 - 2300 rpm). what is the final ratio or what is the 3rd gear ratio. tia

 

[C Schomer]

Diff ratio x trans ratio = final ratio. 3rd is direct or 1(in lockup). So 1 x 3. 55 = 3. 55(FR). And if OD is . 74 it would be . 74 x 3. 55 = FR. If 1st is 5. 5 it would be 5. 5 x 3. 55 = FR and so on. Craig

 

[K_Arts]

What size tires do you have?



I was told that the 3. 5 ratio only applies to factory tires with are 245's. So when you add the 265's you would be around a 3. 42 final ratio.



Correct??



Kev

 

[RMH]

reply to stock tire size

The stock tire size on my truck is 235 85r 16 lr /e

 

[K_Arts]

Ohh geeshh... you guys with the 1 tons always gotta be difficult don't ya??? LMAO.



So then out of curiosity... how do we determine what the final gear ratio is?



Cause I know that the 245's are stock on the 2500's... or at least according the the paper work for both of my trucks.



Hmmm. . inquiring minds would like to know. . LOL



Kev

 

[SS]

Well............

The final drive ratio is just that - the gear reduction multiplication of the components (transmission/transfer case/axle). Tire size is not usually considered, but you are correct to say that it does have some effect.



If you want to compare for 2 different tire sizes, multiply the final drive by the ratio of tire sizes. So for a truck with 245 tires in 3rd gear (1:1) and a 3. 55 axle, the final drive is 3. 55. The same truck with 265 tires has a final drive of 3. 55 or "effective" final drive of 3. 55 x (14. 1" static load radius / 14. 7" static load radius) = 3. 41. For a 235/85 the "effective ratio" would be 3. 55 x (14. 1 / 14. 8) = 3. 38.



My truck with 305/85 tires and 4. 10 gears has the "effective ratio" of a truck with 265's and 3. 55 gears (same engine RPM at the same speed). I hope this isn't too confusing - it seems harder to describe in writing.

 

[Barry]

If memory serves me correctly the auto OD has a final drive of . 69 as opposed to the bolt action trannie's . 75 or . 74. With the advent of the 265 tires, the 3:54 gears and the . 69 OD ratio, it makes for a very tall final drive ratio. It appears the engineers at DC have finally recognized this and are dropping the 3:54 and going to 3:73 as the standard ratio. I wished they offered a 3:90 also... ... ... ... .

 

[srawleigh]

Third gear ratio on an automatic is 1. 00 same as direct drive. Is this what you were asking?

 

[RMH]

I do believe C. Schomer has the answer. thanks to all replys. Always helpful guys on this forum. cia