Got a 1996 fl70 few months back and put it to work, great running/driving truck for my needs, specs are 5.9 cummins mech.IP Allison 3060 6sp trans and 7:17 rear gear, I weigh around 24k loaded and i get 64 mph with eng, wound up tight, about 7.5 mpg i figure, i need advice on a ratio change to get around 70mph at a more comfortable rpm. thanks in advance !
fl70 rear gear advice needed please
- Thread starter barkoguru
- Start date
There are several ratio/rpm converters on the internet. Here's one: Engine RPM / Engine Speed Calculator
There are several others that are more complex. You'll need to measure your tire diameter, and it'd be helpful if you know the transmission output gear ratio in overdrive.
It'd be even more helpful if you could match that to your engine's most efficient rpm. What engine do you have, and what hp level is it rated? From there, a torque/hp graph could be looked up, and a gear selected.
Btw, what's the elevation like where you usually run? Lots of hills, or flat ground?
Edit: I totally overlooked that you'd already posted engine and trans info. I'm gonna start googling.
Edit 2: Cummins has a program that's free to download that apparently will answer this question for you.
http://cumminsengines.com/powerspec
Edit 3: What rpm are you running at 65mph?
Edit 4: Assuming that you are running an 11R-22.5 tire, which are around 41 inches tall when new, and your 6th gear ratio is 0.65:1, and a rear end ratio of 7.17:1, you will be turning almost 2500 rpm at 65mph. Is that correct?
There are several others that are more complex. You'll need to measure your tire diameter, and it'd be helpful if you know the transmission output gear ratio in overdrive.
It'd be even more helpful if you could match that to your engine's most efficient rpm. What engine do you have, and what hp level is it rated? From there, a torque/hp graph could be looked up, and a gear selected.
Btw, what's the elevation like where you usually run? Lots of hills, or flat ground?
Edit: I totally overlooked that you'd already posted engine and trans info. I'm gonna start googling.
Edit 2: Cummins has a program that's free to download that apparently will answer this question for you.
http://cumminsengines.com/powerspec
Edit 3: What rpm are you running at 65mph?
Edit 4: Assuming that you are running an 11R-22.5 tire, which are around 41 inches tall when new, and your 6th gear ratio is 0.65:1, and a rear end ratio of 7.17:1, you will be turning almost 2500 rpm at 65mph. Is that correct?
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You weren't kidding.
Do you have any idea the make and model of the rear axle? Now it's time to figure out what's available.
Around what rpm does this thing generally like to downshift? Like, when does the horsepower start dropping off? You'll want your target cruise rpm higher than that, so if you encounter a slight grade, it doesn't immediately have to downshift.
Depending on the pump's setup (which can vary widely by application), the torque peak is gonna be between 1600-1800, and peak power between 2000 and 2400. I'm gonna say that you'll want to shoot for around 2000-2200rpm at 70 mph, that'll give you some flexibility of you encounter a grade, and a downshift won't be immediately needed. Granted, you might get better mileage at 1800, but that's a pretty tight window before power drops off.
Then again, I'm no expert at this, so other folks might have first hand experience with this that they'd like to share.
Here are the engine rpms at different ratios at 70 mph.
5.0:1=1864 rpm
5.1:1=1902 rpm
5.2:1=1939 rpm
5.3:1=1976 rpm
5.4:1=2014 rpm
5.5:1=2051 rpm
5.6:1=2088
5.7:1=2125
5.8:1=2163
5.9:1=2200
6.0:1=2237
6.83:1=2547
Your 7.1:1 gets you 2647rpm at 70mph.
Depending on the pump's setup (which can vary widely by application), the torque peak is gonna be between 1600-1800, and peak power between 2000 and 2400. I'm gonna say that you'll want to shoot for around 2000-2200rpm at 70 mph, that'll give you some flexibility of you encounter a grade, and a downshift won't be immediately needed. Granted, you might get better mileage at 1800, but that's a pretty tight window before power drops off.
Then again, I'm no expert at this, so other folks might have first hand experience with this that they'd like to share.
Here are the engine rpms at different ratios at 70 mph.
5.0:1=1864 rpm
5.1:1=1902 rpm
5.2:1=1939 rpm
5.3:1=1976 rpm
5.4:1=2014 rpm
5.5:1=2051 rpm
5.6:1=2088
5.7:1=2125
5.8:1=2163
5.9:1=2200
6.0:1=2237
6.83:1=2547
Your 7.1:1 gets you 2647rpm at 70mph.
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It looks like a 5.29, 5.86, 6.83 are available, perhaps more in that range. Looks like they might come in different axle spline counts too. I can't help you there, you'll have to talk to someone who's more familiar.
https://www.heavytruckparts.net/detailview.php?vehicle=53688&vendor=3135155&store=1&searchnum=-1
https://www.heavytruckparts.net/detailview.php?vehicle=66607&vendor=53649&store=1&searchnum=-1
These came off of the first website I clicked on.
https://www.heavytruckparts.net/detailview.php?vehicle=53688&vendor=3135155&store=1&searchnum=-1
https://www.heavytruckparts.net/detailview.php?vehicle=66607&vendor=53649&store=1&searchnum=-1
These came off of the first website I clicked on.
Your gearing is really low. What are the input and output shaft ratios on the transmission? You need to know these as well as the true engine output specs. The weakest expensive link in the drive line is usually the transmission.
I would sit down with an Allison dealer to find out what your best options are. They will find out how much power is coming off the engine and based on the G.V.W. they will recommend the best gear ratio for you to get your RPM's down, It may also be easier to upgrade the transmission.
Im running a 2003 International 4300 S/T 11R22.5 tires 225 HP D.t.466E Non E.G.R. engine, Allison 2000 trans, Merritor rear axle I think the rears are 4.33:1 I can run at 62 mph at 2000 rpm and average 9 MPG. get in lots of wind or steep hills avg MPG drops to 8.
What is your highway speed at 2000 rpm. Most engines are done producing usable power betwene 1900 and 2100 rpm, after that you are wasting allot of fuel for only a few H.P. that arent worth the fuel and wear and tear.
Before thinking big and expensive Start with the basics. Fuel filter needs replacing, dirty, clogged, or weak fuel injectors, or turbos, fuel lines breaking down internaly, fuel pump. Were the valves ever adjusted if so when, and did they set up easily? Did you or the former owner/s install any after market "power adders" like chips or other useless gadgets These are all common fuel and power wasters. Medium duty engines with over 400,000 are often nearing the inframe stage of their life.
Who is the manufacturer of rear axle are you running. Most med duty axles are either Merrior or Spicer. Depending on the manufacturer and age of the drive axle you might be looking at more than a gear swap.
Good luck and keep us up to date.
Bob Wolf.
I would sit down with an Allison dealer to find out what your best options are. They will find out how much power is coming off the engine and based on the G.V.W. they will recommend the best gear ratio for you to get your RPM's down, It may also be easier to upgrade the transmission.
Im running a 2003 International 4300 S/T 11R22.5 tires 225 HP D.t.466E Non E.G.R. engine, Allison 2000 trans, Merritor rear axle I think the rears are 4.33:1 I can run at 62 mph at 2000 rpm and average 9 MPG. get in lots of wind or steep hills avg MPG drops to 8.
What is your highway speed at 2000 rpm. Most engines are done producing usable power betwene 1900 and 2100 rpm, after that you are wasting allot of fuel for only a few H.P. that arent worth the fuel and wear and tear.
Before thinking big and expensive Start with the basics. Fuel filter needs replacing, dirty, clogged, or weak fuel injectors, or turbos, fuel lines breaking down internaly, fuel pump. Were the valves ever adjusted if so when, and did they set up easily? Did you or the former owner/s install any after market "power adders" like chips or other useless gadgets These are all common fuel and power wasters. Medium duty engines with over 400,000 are often nearing the inframe stage of their life.
Who is the manufacturer of rear axle are you running. Most med duty axles are either Merrior or Spicer. Depending on the manufacturer and age of the drive axle you might be looking at more than a gear swap.
Good luck and keep us up to date.
Bob Wolf.
Did some more digging. Gears were available for that rear in the following ratios:
2.80 A41490 1 41490 41769
2.93 A41158 1 41158 41657
3.07 A41494 1 41494 41773
3.21 A41496 1 41496 41775
3.42 B41164 1 41164 41715
3.42 (Q1785 CARRIER) B41530 1 41530 41715
3.58 B41498 1 41498 41777
3.58 (Q1785CARRIER) B41532 1 41532 41777
3.73 B41500 1 41500 41779
3.73 (Q1785 CARRIER) B41534 1 41534 41779
3.91 B41502 1 41502 41781
3.91 (Q1785 CARRIER) B41536 1 41536 41781
4.10 B41504 1 41504 41783
4.10 (Q1785 CARRIER) B41538 1 41538 41783
4.30 A40066 1 40066 40063
4.33 B41266 1 41266 41717
4.33 (Q1785 CARRIER) B41540 1 41540 41717
4.56 B41542 1 41542 41831
4.63 B41268 1 41268 41719
4.88 B41270 1 41270 41721
4.88 (Q1785 CARRIER) B41544 1 41544 41721
5.13 A41272 1 41272 41723
5.13 (Q1785 CARRIER) B41546 1 41546 41723
5.29 B41274 1 41274 41725
5.29 (Q1785 CARRIER) B41548 1 41548 41725
5.57 A41304 1 41304 41747
5.57 (Q1785 CARRIER) B41550 1 41550 41747
5.86 B41456 1 41456 41749
5.86 (Q1785 CARRIER) B41552 1 41552 41749
6.14 A41458 1 41458 41751
6.14 (Q1785 CARRIER) B41554 1 41554 41751
6.43 A41460 1 41460 41753
6.43 (Q1785 CARRIER) B41556 1 41556 41753
6.83 A41462 1 41462 41755
6.83 (Q1785 CARRIER) B41558 1 41558 41755
7.17 A41464 1 41464 41757
7.17 (Q1785 CARRIER) B41560 1 41560 41757
This info was copied directly from here:
http://www.wellertruck.com/lit/Weller_Diff_Catalog_2011.pdf
2.80 A41490 1 41490 41769
2.93 A41158 1 41158 41657
3.07 A41494 1 41494 41773
3.21 A41496 1 41496 41775
3.42 B41164 1 41164 41715
3.42 (Q1785 CARRIER) B41530 1 41530 41715
3.58 B41498 1 41498 41777
3.58 (Q1785CARRIER) B41532 1 41532 41777
3.73 B41500 1 41500 41779
3.73 (Q1785 CARRIER) B41534 1 41534 41779
3.91 B41502 1 41502 41781
3.91 (Q1785 CARRIER) B41536 1 41536 41781
4.10 B41504 1 41504 41783
4.10 (Q1785 CARRIER) B41538 1 41538 41783
4.30 A40066 1 40066 40063
4.33 B41266 1 41266 41717
4.33 (Q1785 CARRIER) B41540 1 41540 41717
4.56 B41542 1 41542 41831
4.63 B41268 1 41268 41719
4.88 B41270 1 41270 41721
4.88 (Q1785 CARRIER) B41544 1 41544 41721
5.13 A41272 1 41272 41723
5.13 (Q1785 CARRIER) B41546 1 41546 41723
5.29 B41274 1 41274 41725
5.29 (Q1785 CARRIER) B41548 1 41548 41725
5.57 A41304 1 41304 41747
5.57 (Q1785 CARRIER) B41550 1 41550 41747
5.86 B41456 1 41456 41749
5.86 (Q1785 CARRIER) B41552 1 41552 41749
6.14 A41458 1 41458 41751
6.14 (Q1785 CARRIER) B41554 1 41554 41751
6.43 A41460 1 41460 41753
6.43 (Q1785 CARRIER) B41556 1 41556 41753
6.83 A41462 1 41462 41755
6.83 (Q1785 CARRIER) B41558 1 41558 41755
7.17 A41464 1 41464 41757
7.17 (Q1785 CARRIER) B41560 1 41560 41757
This info was copied directly from here:
http://www.wellertruck.com/lit/Weller_Diff_Catalog_2011.pdf
A 3:91 or a 3:73, the latter is what i would use, should get you in that rpm range.
I have a 2001 FL70 w/a ISC & 6-speed stick shift & it came w/4.11 gear that would run 70 @ 2400,60 was still 2000 rpm getting 8 mpg empty,I changed it a 3.42 & now I run 63 mph @ 1750 rpm over 10.5 mpg.Speed kils mpg in these trucks esp mine because I have the tall box & have found that it really doesn't make a big difference time wise to slow down but it sure helps the mileage.I ran 70 once for a tank & got 8.5.
Also as you change your gearing you'll also lose hill climbing,I'm real lucky I go to Wyoming on 90 & once in a while depending on the wind will have to downshift & get down to 40 mph in the mountains.That's with maybe 6K in the box,total weight 23K .I'd hate to think how it would be on the side roads where it's alot steeper.I'm lucky that I don't have to.
Oh my ISC has 225HP & 660 Lbs of torque.Is your motor 195,400?
Oh my ISC has 225HP & 660 Lbs of torque.Is your motor 195,400?
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My eng. is the 210hp version not sure on the torque, I don't have a box to worry about, cab is the highest point ever on me, down here in the coastal region I don't have a lot of hills just now and then maybe a hill or two mostly flat hwy and winding backroads and mostly 50 to 100 mile quick runs and the occasional 250 mile trip .
Just curious if that truck was auto or stick, if so do you know what trans it had in it ? thanks
The difference in the gear ratio between auto and stick is the reason you're getting both recommendations for the 3.73 and 5.86 rear end ratios. It HAS to be a stick truck he's mentioning. I'd be curious to know exactly which one so I could look up the ratios for it.