4:10's ????

[Guest00]

what rpm's will an auto turn at 55, 65, 75 mph in od and 3rd ?? I have 3:54's and turn 2,000 at 75 in od and 2,700 in 3rd, peak torque is at approx 2,200 - 2,700, peak hp at 2,700. Towing I gross about 18k. If I want to pull a healthy grade I have no problems at 2,200 in od but that is in excess of 85 mph ( 10 mpg with 1200* egt ) and the police really frown upon that. other wise I end up in 3rd at 2,700 and watch the egt in excess of 1300* and fuel mileage drops well below 5 mpg

What will 4:10's do for me ????

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2K, 2500,Laramie,Quad,4x4,3:54LSD,auto,reese 15k 5th slider, EISLEY BULL BAR,WESTIN NERFS, TA's,9000's, DD2's,PE-EZ & elbow, ISSPRO pyro & boost, Turbo Lifesaver, JARDINE 4", DTT VB,TC and COOLER, PAC BRAKE, MAG HYTECH TP& DC, onboard.
pullee #1 26' Arctic Fox 5th
#2 22' car hauler w/ 76 Pinto 392cid or 68 Cyclone GT Cobra Jet FB
NRA,VFW,SASS

 

[RobG]

I don't know about 55, but at 65 I'm turning right at 2000 RPM and at 75 I'm at 2500 RPM in my ETH, with the Rickson Truck 19. 5" wheels (about 1/8" to 1/4" taller tire than the stock 235/85-16's).

Something you might look into before a gear swap is a US Gear unit set to Underdrive. I think a gear swap on your 4x4 could cost $1k, and the US Gear installed, while costing 2x to 2. 5x more, would give you more gears to play with.

Well okay, financially it's a dumb idea, but it's an interesting attempt at justification. #ad


Rob
2001. 5 3500 QC 4x4 ETH/DEE 4. 10 USGear OD


[This message has been edited by RobG (edited 12-28-2000). ]

 

[HC]

If the gear ratios posted on Fritz's page are correct, and if you get 2000 rpm @75 mph, then when the torque converter is not slipping your rpm's should look like this:

______________3. 54______//______4. 10
Speed_/___3rd__/___OD___//__3rd___/__OD
__55__/__2125__/__1475__//__2450__/_1700
__65__/__2500__/__1725__//__2900__/_2000
__75__/__2900__/__2000__//__3350__/_2325

Sorry about the ____ - I had to do that to make the numbers in the chart stay lined up. These numbers are what the "gear calculator" says. But if you're sure that your 3rd gear rpm at 75 is really 2700, then the above numbers will be a little off. My chart says you should be doing more like 70 mph at 2700 rpm in 3rd. To build a new chart that is calibrated for your truck & tires, drive those speeds, in 3rd and in OD, and multiply your current rpm by 4. 1, then divide by 3. 54. The new number will be exactly what you would get after swapping to 4. 1 gearing.

I think if I were in your shoes, I'd stay with the 3. 54's, and drive with the OD off. You'll pay a little in fuel, but the 3rd gear is probably much stronger and you'll get back your money in longer life. It would take a lot of miles with improved mpg to pay back cost of the swap, and you'd still be putting more strain on that relatively weaker OD gearing than the 3rd gear + 3. 54 combo. If you use the OD anyway even with 3. 54's, then 4. 10's would be the ticket to reduce the strain. You might want to try this question in the new "Towing" forum - I'm basing my answer on a belief in the strength of 3rd gear, but I don't have heavy towing experience. One final thought - since you show injectors in your signature, I'd take those mpg readings while pulling heavy hills with a huge grain of salt. I'm sure as you get to higher power levels the injectors would cause greater inaccuracies in the trip computer mpg. So you may be doing better than you think in the "OD off" hill climbs, at least when you're not accelerating up the hill!

[This message has been edited by HC (edited 12-28-2000). ]

 

[Barry]

If the 4:10s seem a little tight, do the math on 3:73s or 3:90s. Given a choice, I would order my truck with 3:73s. The autos are 5 pts. taller than the manuals. Maybe 3:90s would be better. I was quoted $350 for the R&P set that would slide right in my carrier.

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2001 2500 QC, ETH/DEE,4:10s, LSD, VA Magic Box, Boost/Pyro Guages, 4" Turbo back, BD Brake,Mag- Hytec Cover, Kleen Wheels, Energy Release, 70 gal. fuel, 20K Reese. 22,000 miles on 12/19/00 (RV Dealer)

 

[Vaughn MacKenzie]

Jack, (off the subject) get your fuel lines installed yet? Haven't heard from you for quite awhile and wasn't sure if you got them. My email's not working (won't send but will receive), lemme know.

Vaughn

Vaughn_MacKenze@KPHD.org

 

[MikeR]

Hey HC,
I think the reason why the RPM's on the chart are not comparing to JNutters',is that the chart on the Fritz site was made with 245/75/16 tire sizes on a truck. JNutters' truck has 265/75/16 from the factory,and the taller tire makes the engine turn exactly 200rpm slower... just like what this thread has shown.

Jack,if it's too expensive to swap to 4. 10's,maybe you'd consider putting a 285 or 295/75/16 tire on your truck and tow in 3rd gear. This will bring you down another 150-200rpm.

-Mike

 

[Guest00]

Thanks for the replies. #ad

the rpm #'s are approx. as I think the 2700 was actually 73 or 74 mph. at 2800 rpm she started loosing power so I would run @ 2700. The problem with running at 2700 was egt. It has the hp to pull at 2700 in 3rd but not enough torque at 1600 - 2000 in od to pull the same grade.

 

[HC]

I think the reason why the RPM's on the chart are not comparing to JNutters',is that the chart on the Fritz site was made with 245/75/16 tire sizes on a truck. JNutters' truck has 265/75/16 from the factory...

Mike,
I need to clarify my question: Are the transmission ratios reported by Fritz still correct for the 2000 year trucks? http://www.ecpe.vt.edu/~dfritz/technical/Ramspecs.htm
I used the gearing of 1. 0 in 3rd, 0. 69 in OD, reported for "1994 up" transmissions. Has this changed in recent model years?
Using those ratios, plus the 3. 54 rear end, plus my own tire diameter, which is also 265/75R/16, I was right on the money with JNutter's reported 75 mph at 2000 rpm, but, if the year 2000 transmission is still 0. 69/1. 0 (OD/3rd), then that should rise to 2900 in third at the same speed, vs. the 2700 JNutter reported.
P. S. - I like your simple yet effective solution: Bigger tires! If JNutter has difficulty finding the load range he wants, this could even be used as an excuse to go to the Ricksons. #ad