4.56 axle ratio

[mpaulson]

Has anyone done a swap to 4. 56 axle ratio? I'm concidering it since this would put my RPM's at about 2250 at 110km/h (70mph), right at peak towing performance RPM's. The 4. 10 seems to low, I'd be at 1900RPM's and if I shifted to 5th, then I'd be at 2500 RPM's which is to high for my liking.

I've started looking into an underdrive unit, but it doesn't appear that anyone is making an accual underdrive. Gear Vendors only makes an over drive and not one for the 68 rfe. An under drive unit would get me the best of both worlds.

Any advice would be appreciated.

 

[Schrowang]

In my 04. 5' I never had a problem towing with the 4. 10 and the NV5600 transmission. Towed a 30' fiver all over the mountain passes in CO and it never gave the impression that it was lugging. I used 3rd @ 35mph all the way up Wolfcreek on US-160 and it never burped once. On the highway I could run in 6th unless I hit a good hill and then a shift down to 5th usually took care of the hill.

 

[Simplysmn]

4;56 would be way off I would stick with the 410's better towing gear

 

[Alan Reagan]

I would have thought that the 4. 10 would have gotten you about 2200-2250 at 70 mph. 4. 56 is a pretty low gear. That's what my 01 was and that's the rpms I remember. But, the tires might have been 16 inch.

 

[RustyJC]

I would have thought that the 4. 10 would have gotten you about 2200-2250 at 70 mph.

With the double overdrive 68RFE in 6th gear and 4. 10s (on my 3500 dually):



60 MPH = 1612 RPM



70 MPH = 1880 RPM



The 4. 10s in conjunction with the 68RFE are, AFAIC, an ideal setup for heavy towing and running empty. My 2002 3500 dually with the NV5600 and 4. 10s was spinning 2000 RPM @ 60 MPH and 2350 RPM @ 70 MPH - OK for towing, not much fun running empty on the Interstate. The difference is that the NV5600 was only a single overdrive transmission - the 68RFE's double overdrive provides that mysterious "7th gear" I was always reaching for when running empty with the 2002 truck.





Rusty

 

[mpaulson]

My wonders are if the benifit of the splitting the gears by having the 6th gear running me at 2300rpm at 70 mph which is my normal crusing speed while traveling would be worth it. The 4. 56 would get me to the same rpms in sixth gear as 5th gear does right now. I use the spread sheet that I have attached a link to below. You can change the gear ratio in the box at the top on the appropriate transmission and you will get your speeds per gear/rpm.

I have never been able to compare the difference from one truck to the next for the different ratios, does it make accellerating that much less stressful on the drivetrain? I do about 12,000 kms of RV hauling a year and about 70% of that is in the mountains.



http://www.klenger.net/dodge/general-reference/gear-speed-calculator.xls

 

[HBarlow]

I have pondered the 4. 56 gear many times but don't tow enough now to warrant the expense and the sacrifice of giving up the great highway cruise capability of the 4. 10 gear.

If I was a full timer towing all the time with my truck I would look into the availability of the 4. 56 gears for my AAM differential.

The higher numerical/lower gearing will always provide a little more starting torque and a little bit faster acceleration and with the six speed automatics there would be very little penalty at higher road speed.

It is wind and load that decreases fuel economy, not rpm.

 

[wjpatton]

I agree with Harvey on this one. 10 knots of headwind or more just kills mileage at any speed above about 60 mph. Driving across TX today I had to run about 71 mph (GPS) to keep in 6th gear. Any slower and the speed just kept decreasing until I downshifted to 5th (68RFE/3. 73). Earlier in the week with a nice tailwind, I was pulling down 11+ mpg at 73 mph!!!! Don't ask what it was today!!!!

 

[AH64ID]

If your always towing then it's probably a good idea rpm wise. 6th gear is the weakest gear of the trans thou, and you may want to just use 1-5 with 4. 10's and keep 6th for empty towing.

 

[NIsaacs]

An underdrive unit of any kind will not work well with a 6spd ratios. 6th under would be about the same as 5th direct, 5th under would be the same as 4th direct and so on. It is hard to get a perfect gear for all driving conditions however I would think 4. 10's would be almost there.



Nick

 

[HBarlow]

An underdrive unit of any kind will not work well with a 6spd ratios. 6th under would be about the same as 5th direct, 5th under would be the same as 4th direct and so on. It is hard to get a perfect gear for all driving conditions however I would think 4. 10's would be almost there.

Nick

I agree. The 4. 10 ratio is as good as it gets with the six speed automatics unless someone goes to the trouble and expense to locate and install a gear set not offered by Ram.

 

[RustyJC]

If you're wanting 2250 RPM @ 70 MPH, why not just drop into 5th?



Rusty