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 > How critical is axle ratio?

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kellem

Shenandoah valley,VA

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Posted: 06/16/21 07:26am Link  |  Quote  |  Print  |  Notify Moderator

Hemling wrote:

It does concern me a bit, and I'm thankful for this discussion, because in the near future I hope to be a 2022/2023 (whenever I get around to it) super duty owner. Right now towing a 9K TT, and probably something like a 12-13K fifth wheel in the future. for me, I've made the decision that a 7.3 F-350 makes the most sense. I don't want a truck that turns too many RPMs unloaded on the highway, but I also don't want it to lug while towing. Anything will seem like a huge improvement over my 4r100/3.73 combo right now. I guess it's first world problems when you get down to it. Anybody running 4.30s with a newer 7.3/10 speed, what are your RPMs on the interstate? Does it feel like it's too high? Anyone running 3.55 or so gears? Do you even use 10th?!?


Have the 7.3 with 3.55 gears and while towing a 30ft 7600# trailer at 65mph on the interstate, I always lock-out 9-10.
Truck runs at 2100 RPMS and rarely downshifts.
Empty getting 17.5 mpg highway.
Towing-10.5 highway.

Terryallan

Foothills NC

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Posted: 06/16/21 07:29am Link  |  Quote  |  Print  |  Notify Moderator

deltabravo wrote:

Terryallan wrote:

So if you have one of the makes we made axles for..

Who is this "We" you speak of?


When I started. It was GETRAG GEAR. A German company. Then years later, we were sold to Dana Axle. a few years later, GETRAG bought us back. Until finally we were sold to GKN Driveline.

and recently there was a hostile take over of GKN, by Melrose. We were then to be sold again. But as of now. No buyer can be found. And it is still called GKN.


Terry & Shay
Coachman Apex 288BH.
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Hemling

Miami, FL

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Posted: 06/16/21 09:54am Link  |  Quote  |  Print  |  Notify Moderator

kellem wrote:

Hemling wrote:

It does concern me a bit, and I'm thankful for this discussion, because in the near future I hope to be a 2022/2023 (whenever I get around to it) super duty owner. Right now towing a 9K TT, and probably something like a 12-13K fifth wheel in the future. for me, I've made the decision that a 7.3 F-350 makes the most sense. I don't want a truck that turns too many RPMs unloaded on the highway, but I also don't want it to lug while towing. Anything will seem like a huge improvement over my 4r100/3.73 combo right now. I guess it's first world problems when you get down to it. Anybody running 4.30s with a newer 7.3/10 speed, what are your RPMs on the interstate? Does it feel like it's too high? Anyone running 3.55 or so gears? Do you even use 10th?!?


Have the 7.3 with 3.55 gears and while towing a 30ft 7600# trailer at 65mph on the interstate, I always lock-out 9-10.
Truck runs at 2100 RPMS and rarely downshifts.
Empty getting 17.5 mpg highway.
Towing-10.5 highway.



So, when you are not towing, does it use 9th and 10th? What RPMs and MPHs do you see then?

Grit dog

Black Diamond, WA

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Posted: 06/16/21 10:47am Link  |  Quote  |  Print  |  Notify Moderator

Hemling wrote:

kellem wrote:



Have the 7.3 with 3.55 gears and while towing a 30ft 7600# trailer at 65mph on the interstate, I always lock-out 9-10.
Truck runs at 2100 RPMS and rarely downshifts.
Empty getting 17.5 mpg highway.
Towing-10.5 highway.



So, when you are not towing, does it use 9th and 10th? What RPMs and MPHs do you see then?


They could be used downhill or with a stiff tail wind. Otherwise 9 & 10 are not useful or efficient gears for any sizeable trailer towing.
Especially with high gears like 3.55s and the bigger diameter tires that are common on newer trucks.


2016 Ram 2500, MotorOps.ca EFIlive tuned, 5” turbo back, 6" lift on 37s
2017 Heartland Torque T29

BenK

SF BayArea

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Posted: 06/16/21 11:19am Link  |  Quote  |  Print  |  Notify Moderator

Hemling wrote:

snip...

So, when you are not towing, does it use 9th and 10th? What RPMs and MPHs do you see then?



to answer this, you will need to provide some contextual info...like your engine torque/HP curve, what truck/trailer/etc, terrain and how fast you wish to accelerate and constant speed you will maintain in the terrain you wish to travel

Close ratio tranny's provides more gear ratios to keep your engine at the optimum RPMs at whatever speed you are trying to maintain.


-Ben Picture of my rig
1996 GMC SLT Suburban 3/4 ton K3500/7.4L/4:1/+150Kmiles orig owner...
1980 Chevy Silverado C10/long bed/"BUILT" 5.7L/3:73/1 ton helper springs/+329Kmiles, bought it from dad...
1998 Mazda B2500 (1/2 ton) pickup, 2nd owner...
Praise Dyno Brake equiped and all have "nose bleed" braking!
Previous trucks/offroaders: 40's Jeep restored in mid 60's / 69 DuneBuggy (approx +1K lb: VW pan/200hpCorvair: eng, cam, dual carb'w velocity stacks'n 18" runners, 4spd transaxle) made myself from ground up / 1970 Toyota FJ40 / 1973 K5 Blazer (2dr Tahoe, 1 ton axles front/rear, +255K miles when sold it)...
Sold the boat (looking for another): Trophy with twin 150's...
51 cylinders in household, what's yours?...

kellem

Shenandoah valley,VA

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Posted: 06/16/21 09:01pm Link  |  Quote  |  Print  |  Notify Moderator

Hemling wrote:

kellem wrote:

Hemling wrote:

It does concern me a bit, and I'm thankful for this discussion, because in the near future I hope to be a 2022/2023 (whenever I get around to it) super duty owner. Right now towing a 9K TT, and probably something like a 12-13K fifth wheel in the future. for me, I've made the decision that a 7.3 F-350 makes the most sense. I don't want a truck that turns too many RPMs unloaded on the highway, but I also don't want it to lug while towing. Anything will seem like a huge improvement over my 4r100/3.73 combo right now. I guess it's first world problems when you get down to it. Anybody running 4.30s with a newer 7.3/10 speed, what are your RPMs on the interstate? Does it feel like it's too high? Anyone running 3.55 or so gears? Do you even use 10th?!?


Have the 7.3 with 3.55 gears and while towing a 30ft 7600# trailer at 65mph on the interstate, I always lock-out 9-10.
Truck runs at 2100 RPMS and rarely downshifts.
Empty getting 17.5 mpg highway.
Towing-10.5 highway.



So, when you are not towing, does it use 9th and 10th? What RPMs and MPHs do you see then?


When not towing 9th and 10th engage as normal when speed Dictates.....the higher gears are all influenced by speed.

4x4ord

Alberta

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Posted: 06/17/21 06:01am Link  |  Quote  |  Print  |  Notify Moderator

I think choosing the best gear ratio begins with having a good understanding of your engine’s powerband. Seeing a torque curve of the 7.3 would be very useful. A bsfc map would also be helpful. After getting a sense of your engine’s capabilities knowing the transmission’s gear ratios is essential. Finally knowing how the truck will be used needs to be considered.
I don’t know much about the 7.3 other than it has a long flat torque curve and I believe it makes 430 peak hp at 5500 rpm. It makes 425 lb ft of torque at 3250 rpm (263 hp). and 350 hp at 3900 rpm.

350 hp should be more than enough power to tow a 12000 lb rv up a 6% grade at 60 mph.

With 3.55 gears and the 10 speed 7 th gear would put you at 2150 rpm at 60 mph with 34 inch tires and 4th gear would put the engine at 3800 rpm at 60 mph.

Without giving it much more thought I’d agree with Shiners earlier post:
If your going to be running empty a fair bit get 3.55 gears and you’ll have the opportunity to get better fuel economy unloaded on the highway. While towing you can lock out the top gears and still have all the performance you need, seldom dropping below 4th gear in the hills.

If your primarily going to be towing with the truck you may as well get 4.30 gears to gain a little bit of grunt at the rear wheels to get your trailer moving in first gear. The odd time when your running empty down the highway you’ll be at 2060 rpm in 10 th gear at 75 mph…. which doesn’t sound too bad.


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Crespro

SoCal

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Posted: 06/17/21 12:28pm Link  |  Quote  |  Print  |  Notify Moderator

4x4ord wrote:

I think choosing the best gear ratio begins with having a good understanding of your engine’s powerband. Seeing a torque curve of the 7.3 would be very useful. A bsfc map would also be helpful. After getting a sense of your engine’s capabilities knowing the transmission’s gear ratios is essential. Finally knowing how the truck will be used needs to be considered.
I don’t know much about the 7.3 other than it has a long flat torque curve and I believe it makes 430 peak hp at 5500 rpm. It makes 425 lb ft of torque at 3250 rpm (263 hp). and 350 hp at 3900 rpm.

350 hp should be more than enough power to tow a 12000 lb rv up a 6% grade at 60 mph.

With 3.55 gears and the 10 speed 7 th gear would put you at 2150 rpm at 60 mph with 34 inch tires and 4th gear would put the engine at 3800 rpm at 60 mph.

Without giving it much more thought I’d agree with Shiners earlier post:
If your going to be running empty a fair bit get 3.55 gears and you’ll have the opportunity to get better fuel economy unloaded on the highway. While towing you can lock out the top gears and still have all the performance you need, seldom dropping below 4th gear in the hills.

If your primarily going to be towing with the truck you may as well get 4.30 gears to gain a little bit of grunt at the rear wheels to get your trailer moving in first gear. The odd time when your running empty down the highway you’ll be at 2060 rpm in 10 th gear at 75 mph…. which doesn’t sound too bad.


4x4ORD has good insights. With my truck and 15K fifth wheel, the 3.55 Ford engineer limits of 14.8K and 21.8K are too low. The 4.30 has Ford limits of 18.9K and 26K. These provide a reasonable comfort margin (about 25% and 15%). IMHO, the OP would be happier in the long run with greater towing margin.


Crespro 2021 Grand Design 310GK-R, 2020 F250LB, 7.3L, 4.30, Reese 27K

blt2ski

Kirkland, Wa

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Posted: 06/17/21 05:46pm Link  |  Quote  |  Print  |  Notify Moderator

Issue with just going by axle ratio to get your GCWR per X brand engineers. Is you in reality may be wanting a different speed - 35-40mph depending upon gvwr of tow rig, max start grade - 12%, to name two specs in the current jsae tow spec.
I personally want closer to 30% or greater in 1st gear, and my states min speed of 40mph on a freeway grade. Speed is HP spec, grade starts imity is overall low gear plus a tire radius factor times torque. So if you go strictly by GCWR manufactures give you, your drivetrain may or may not be enough for your needs a d wants.
My 02 on the disCUSSion.

Marty


92 Navistar dump truck, 7.3L 7 sp, 4.33 gears with a Detroit no spin
00 Chev C2500, V5700, 4L80E, 4.10, base truck, no options!
92 Red-e-haul 12K equipment trailer

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