Gear Ratio Analysis & Discussion -- Manual transmission

[67guzzler]

Hi guys. I wanted to throw some facts at ya, some number crunching and then some thoughts. This analysis is for everyone but assume you're shifting for yourself and have a G56 (only because I used the G56's ratios below) and you can control the gear selection for your rig at any time.



Here we go -- The stock tire diameter for the SLT or Laramie truck with 265/70-17 Michelin tires is 31. 4". The stock (only avail for the G56 now) rear is 3. 73 and the gear ratios for the G56 (for this example) are First 6. 29:1; Second 3. 48:1; Third 2. 10:1; Fourth 1. 38:1; Fifth 1. 00:1; Sixth 0. 79:1 respectivey.



3. 73 (stock) rear end and an RPM of:



800 in Lst gear you're going 3. 18 mph.

800 in 2nd gear you're going 5. 76 mph. (most common launch gear)

2200 in 5th gear you're going 55. 09 mph.

2200 in 6th gear you're going 69. 74 mph.



* Nice 1700 RPM Cruise Speed in 6th: 53. 89 mph.



3. 54 rear end and an RPM of:



800 in Lst gear you're going 3. 36 mph

800 in 2nd gear you're going 6. 07 mph

2200 in 5th gear you're going 58. 05 mph.

2100 in 6th gear you're going 70. 15 mph.



* Nice 1700 RPM Cruise Speed in 6th: 56. 78 mph.



3. 08 rear end and an RPM of:



800 in Lst gear you're going 3. 86 mph.

800 in 2nd gear you're going 6. 97 mph.

2200 in 5th gear you're going 66. 73 mph.

1825 in 6th gear you're going 70. 07 mph.



* Nice 1700 RPM Cruise Speed in 6th: 65. 27 mph.



2. 73 rear end and an RPM of:



800 in Lst gear you're going 4. 35 mph.

800 in 2nd gear you're going 7. 86 mph.

2200 in 5th gear you're going 75. 28 mph.

1600 in 6th gear you're going 69. 31 mph.



* Nice 1700 RPM Cruise Speed in 6th: 73. 63 mph.



Everything after this point is mere opinion and brainstorming and is just for the sake of quality discussion. It's merely for academic purposes.



Why on God's Green Earth wouldn't I want to have any of alternative rear end ratios if:



1) I have the G56 stick.

2) Can launch from L if I have to and actually put it go good use.

3) Have a 4x4 and can go into 4WD Low if necessary

4) I want to actually make the 6 speed more like a six speed and not a 4 1/2 speed (3. 48 launch sucks because you're getting out of it almost immediately).

5) I'd like to save some natural resources.

6) I'd like to utilize the spirit and core value of the I-6 CTD -- the low end grunt and save myself from having to overshift like crazy.

7) I have a brain and can pick the appropriate gear.



Anyone have any thoughts? The premise here is that the G56 with a 3. 73 on an engine that loves to run between 1700 and 2000 rpm efficiently is a horrible mismatch. It's a crime to make this engine run 2200 rpm's at 70 mph when it makes gobs of torque practically right off idle. Is my way of thinking way off?



Can one even get a set of 3. 08 gears for the front and rear 2005 CTD 4x4?



Thanks guys. I'd really to appreciate your thoughts on this one -- especially some of the old-timers.



P. S. I used this MPH Calculator based on RPM, Gear Ratio and Rear Ratio



P. P. S. Check out the Gear Vendors Under/Overdrive too. Go to the bottom of the page for the NV5600 application. I emailed them regarding the G56. This interests me as a possible solution.

 

[J&LRam]

rpm's for best hp/torque for towing my 11200 lb 5er is 2200-2400 on my truck. Dodge makes these trucks to work and mine seems happiest at those rpm's. I sure wish I had the G56 OD ratio in my NV5600.

JIM

 

[67guzzler]

rpm's for best hp/torque for towing my 11200 lb 5er is 2200-2400 on my truck. Dodge makes these trucks to work and mine seems happiest at those rpm's. I sure wish I had the G56 OD ratio in my NV5600.

JIM

That's fine. I can't argue with that. But I just want another gear on the top end for the times that the truck is riding unloaded. BTW -- It seems we both got the wrong trucks! You've got a 3. 73 rear and . 73 final drive ratio. You should have the 4. 10 rear. I've got the . 79 final drive ratio & the 3. 73 rear -- effectively the 4. 10 ratio with the NV5600. If that's how you feel, why didn't you get a truck with the 4. 10?

 

[klenger]

This would have saved you some work: http://www.klenger.net/dodge/general-reference/gear-speed-calculator.xls

 

[Tim]

That's fine. I can't argue with that. But I just want another gear on the top end for the times that the truck is riding unloaded. BTW -- It seems we both got the wrong trucks! You've got a 3. 73 rear and . 73 final drive ratio. You should have the 4. 10 rear. I've got the . 79 final drive ratio & the 3. 73 rear -- effectively the 4. 10 ratio with the NV5600. If that's how you feel, why didn't you get a truck with the 4. 10?

I have covered this several times before but final drive ratio = highest available gear multiplied by axle gearing. . 73 x 3. 73 = 2. 72:1 and . 79 x 3. 73 = 2. 95:1 just to do some examples for you.

 

[Nate]

Get some bigger tires.

 

[NIsaacs]

JGann, I agree with your wanting higher gears for the Cummins. I have always said that a gear ratio of about 3. 00, 33" tires and a direct drive 6-spd would be perfect. Since overdrive is the weak link in any transmission, direct would last forever. Even the little 4-spd's of years past would last for years because of this feature, (direct drive), high gear is the gear we all use the most.



Guys wanting 4. 10's with the 6-spd makes no sense to me. It's like they feel if they have to down shift out of 6th on a hill they don't have a tough truck. If you have 3. 73's or 3. 55's and you pull a load up some big hills, drop that puppy into 5th (direct drive) and pour the coal's to'er!



The early trucks with the Getrag worked the best with the 3. 54's, it forced you to downshift more on a hill with a load. The 4. 10 trucks had a lot more trouble with this transmission, they could pull most hills in overdrive, the weak link. My '91' has 293,000 miles, with the original trans, standard oil fill, not over filled by one quart, and 3. 54's.



I don't think the Gear venders will split very good with the close ratio 6-spd, however it will get you a nice low rpm overdrive. Keep in mind you must use direct with an exhaust brake or if you are in 4x4.





"NICK"

 

[67guzzler]

Tim -- Thanks for the math help. That wasn't really the point of the post tho. What I was trying to propose was that the CTD, with all it's torque right off idle and throughout the RPM range was asked to turn unnecessary rev's at the top end. Unloaded & leaving a stop light I'm in 4th gear before I get through the intersection! Great for hauling 40,000 lbs, but crazy otherwise. Basically -- I'd like two more gears on the top end.



Nate -- Bigger tires would be cool and I've actually been playing with the idea of going to some 285's. But the more unsprung weight you have, the rougher the ride. I also really like the factory Michelin 265's the truck came with so it would seem a shame to bail on those right out of the chute.



Nick -- That explains why they say that having the Gear Vendors or the US Gear overdrives will allow the factory transmission to run cooler -- just keep the factory transmission out of OD and a 1:1 ratio and let the auxiliary OD do the "multiplying. " Very interesting.

 

[bkroupa]

I don't tow as fast or heavy as some of you guys, but with 33" tires, 3. 54 gears & NV4500 I have a much better tow/empty compromise than I did before with the 4. 10's in the same truck. With the 4. 10's & 245/75-16's (stock) the truck towed heavy well, never needed to be downshifted from 5th but was screaming all the time & got 19-20 mph running 65 mph empty. I drive 700-800 miles empty a week so I tried 285/75's it it helped with the noise some, but the mpg only picked up ~. 5mpg, no ill effect on towing at all. I swapped out rear axles to 3. 54's & the truck is MUCH more pleasant to drive empty & on flatlands towing. I regularly get 22 mpg + for my normal commute & the engine noise is much quieter/peaceful. OD should be for running empty or flatlands only loaded.

 

[bpenrod]

I have a 200+ mile commute everyday for the job I'm on now and really enjoy the extra gear my GV overdrive provides. It did not seem to increase mpg any but I do cruise faster with it than without it. I used to set the cruise at 73 ish and was getting around 20 mpg. With the GV I use 5 Over and set the cruise at 78 ish. I'm now getting about 21 mpg so not much improvement there but I love cruising down the road at 80 with the engine turning about 1800 rpm. Really quiets the ride down!!

 

[LDobie]

Couldn't agree with you more JGann, it has gotten to the point that if you have to downshift, you don't have the right gearing or enough power. I'm kinda in the same boat as you, I want the "big hammer" 3/4 Ton, but don't want the Hemi, love the power of the CTD, but don't haul very much. Awhile back I made a spreadsheet comparing gear ratios and I liked the 3. 20 very much. Bigger tires won't help with efficiency, as you usually have to go wider to go taller.



I also agree with the other gentleman's thoughts, that a 1:1 gear matchup is the strongest for humping the big loads up the hills. Too bad the aftermarket doesn't make a 3. 54, 3. 43 or 3. 20 gear set for the 3rd Gens

 

[67guzzler]

After some research I just received a quotation from Broward Powertrain for the Gear Vendors unit installed -- $3,600. 00. The guys there who do the installs like these over the US Gear units. They admit they're only functional in 2wd and if you plan to use an exhaust brake, the GV unit should be turned off.



The unit retails for $2,995. I think the price is fair. I just don't know if I'm ready to pull the trigger. The idea of a 1700 rpm 70 mph cruise vs. a 2250 rpm 70 mph cruise is tempting. But $3,600 is a bunch of coin.....

 

[Vaughn MacKenzie]

If I had to get the G56 transmission, make mine a 4x2 with 3. 08 gears. Great fast highway cruiser, and when towing a big load I can use 5th gear. Puts the RPM right where I'd want it driving around 65 a,d the transmission runs cooler and happier in 5th.



Vaughn

 

[Blakers]

Nate -- Bigger tires would be cool and I've actually been playing with the idea of going to some 285's. But the more unsprung weight you have, the rougher the ride. I also really like the factory Michelin 265's the truck came with so it would seem a shame to bail on those right out of the chute. .

I'm very happy with the gear combination I have now since going with 315 tires. I cruise at 72 mph @ 2000 rpm even (that's with the 5600). The ride with the 315s is much better than with the stock 265 Rugged Street tires. I've heard of guys selling their new tires and getting a good price on them, or some tire dealers giving a good trade in on them.



Blake

 

[bpenrod]

I'm happy with my GV, the price is hard to swallow so I had a new drive shaft made up and kept the original transfer case tailshaft. Cost me and extra hundred but now when I sell the truck I have the option of putting the stock pieces on and saving my investment. Should only take an hour tops to swap it back to stock then putting it in the next truck just requires the correct adapter. Most of the highway crusing here in MI is in the 75 to 80 mph range and I love the extra OD gear. 1850 rpm is about 80 mph and 70 to 800* egt's with zero to two pounds boost. Also gets about 21 mpg doing that speed. I have a two hundred plus round trip freeway run for work and can't think of doing it without the GV at the speeds I'm used to now!!

 

[DieselMinded]

http://www.3d.otisnet.net/gearratio.xls

 

[nps]

315's and a 3. 54 rear with a 5600 work well for me. A good cruiser unloaded, and when towing heavy I want to drop out of OD and into direct drive to get the stress off the OD gear anyway.



Had 265s from the factory. Before the 315's went on I thought launches were way too low - second gear was too short and 3rd too tall. Now everything is good. Even find myself launching unloaded in 1st if I'm going uphill.

 

[crobertson1]

Guys wanting 4. 10's with the 6-spd makes no sense to me. It's like they feel if they have to down shift out of 6th on a hill they don't have a tough truck. If you have 3. 73's or 3. 55's and you pull a load up some big hills, drop that puppy into 5th (direct drive) and pour the coal's to'er!

"NICK"

Yeah, that's why I got a 6 speed and 4. 10s, to feel like I got a "tough truck". Try towing in the mountains. I wanted all the useable power I could get, and I got it. At 70 mph, the engine is at about 2300 rpm. I wouldn't want to tow our TT any faster than that anyway.

 

[JeffJackson]

I'm glad I got the 4. 10s on my truck. It's all it can do to pull 5th gear up a 6% grade with the trailer in tow. If I had 3. 73s, I'd be down to 4th or looking at smaller tires (285s or back to stock).

 

[robert4570]

"I'm very happy with the gear combination I have now since going with 315 tires. I cruise at 72 mph @ 2000 rpm even (that's with the 5600). The ride with the 315s is much better than with the stock 265 Rugged Street tires. "

Blake



I'll second that, going from a rather tall 3:54's before .

3:73'S fill the gap nicely between 4:10's which seem like overkill when empty to wishing you had a more reduction when pulling a load with 3:54's.

I was pleased when i seen the specks for the new rear ends.