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Diff gear ratio?

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tj300
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Diff gear ratio?

Post #1 by tj300 » Sun Mar 29, 2009 6:18 pm

I,m trying to determine the differential gear ratio in my'80 F100 short bed.

Here's what's on the differential ID tag,
DM-CJ0F09
75 9 5314B

What does this mean? Any thoughts?
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Post #2 by TCIC 300ci superbeast » Sun Mar 29, 2009 7:34 pm

i think your missing a number just before the 75. my guess is it should be 3.75 if there is an L between the 3 and the 75 then its a track-loc
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Post #3 by StrangeRanger » Sun Mar 29, 2009 7:41 pm

It could also be a 2 which would explain some of the performance issues you've complained about

Did you jack up a rear wheel and check the ratio like I explained to you?
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Post #4 by tj300 » Sun Mar 29, 2009 8:25 pm

I scraped off more dirt from the tag. It was hiding a 2.

The ratio is 2.75. Not a good combination. I'll never get around that gear.

The 300 does not perform well with tall gears and auto tranny.
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Post #5 by ludwig » Sun Mar 29, 2009 8:45 pm

Hunh? What are you trying to get? Over 20 mpg? Or over 60 in ten seconds.
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Post #6 by StrangeRanger » Sun Mar 29, 2009 8:52 pm

Your theoretical speeds in gear (3800 RPM) with 235/75-15 tires are 48/78/115 with a highway RPM of 2150 @ 65 MPH. That's not a package for drag racing or towing but it should give very good highway mileage.

The problem is that if you change the rear gears, the highway RPM increases
2.75 = 2150 @ 65
3.00 = 2350 @ 65
3.25 = 2540 @ 65
3.50 = 2740 @ 65
3.75 = 2930 @ 65

You know your tolerance for RPM better than I do. You pick.

Another option is the wide ratio gearset kit for the C6 from Ford Racing, but it's not exactly cheap, somewhere north of $500 by the time you get all the other standard parts that aren't included in the kit. It changes your ratios from 2.40/1.47/1.00 to 2.72/1.54/1.00. That gives you the same overall reduction in 1st as your present tranny with a 3.11 rear but doesn't affect your highway RPM. That's a more substantial difference than it sounds like.

You still have to resolve your tuning issues before any amount of new parts are going to help.
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Post #7 by Lazy JW » Sun Mar 29, 2009 9:31 pm

tj300 wrote:I scraped off more dirt from the tag. It was hiding a 2.

The ratio is 2.75. Not a good combination. I'll never get around that gear.

The 300 does not perform well with tall gears and auto tranny.


My 81 F-150 has the same ratio and a T-18. I normally start out in second; it isn't a very good drag racer but I have to be careful to keep from spinning the tires. That is with a stock YF single barrel carb and stock single exhaust.
Joe
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Post #8 by StrangeRanger » Sun Mar 29, 2009 9:44 pm

JW:
FWIW your overall gear reduction in 1st is 2.73 x 6.32 = 17.25:1 and in 2nd it's 3.09x 2.73 = 8.44:1

His reduction in 1st with a 2.2 torque converter is 2.2 x 2.4 x 2.73 = 14.41:1

So he should have a net torque to the wheels somewhere between that which you have in 1st and in 2nd. For towing I think I'd want more final drive reduction and for racing, I think I'd want something 1000 lbs. lighter with better aero (but that's just me.) It still should not be a stone if it's tuned right and with the Edelbrock 4v on a DP it should produce some spectacular highway MPG.
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Post #9 by TCIC 300ci superbeast » Sun Mar 29, 2009 9:44 pm

that old 85 f150 i had came with2.75 and a 4speed OD, it was killer on mileage, wasnt to bad on the acceleration, but after i swaped for the 3.50 it was a lot better.

the 9 after the ratio is stands for 9" isnt it.
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Post #10 by StrangeRanger » Sun Mar 29, 2009 9:51 pm

A 2.75 and an OD tranny isn't a real good combination. A 3.50 works much better with the approx 0.80 OD ratio in the various T170 trannies. A 3.50 with the OD is the same as a 2.80 final with a non OD. Just about perfect for most uses.
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Post #11 by American Thunder » Sun Mar 29, 2009 9:56 pm

If you think the 300 is bad with 2.75s and the auto, you should see what a 302 does in that situation. Especially with loud mufflers on it, so the whole neighborhood can hear the poor thing trying to accelerate..
I speak from personal experience!

Adjust the timing and fuel, and that 300 will work a lot better for you, but a swap to a manual trans sure wouldnt hurt.
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Post #12 by Lazy JW » Sun Mar 29, 2009 11:48 pm

StrangeRanger wrote:A 2.75 and an OD tranny isn't a real good combination......


Should an SROD in good condition suddenly fall into my lap I would swap it into my 81 in hopes of better fuel economy, but that thing isn't expected to do the work of my White Ox either. I wouldn't spend much money on it though because the ROI would be fairly small. First gear in the SROD is a tad lower than 2nd in my T-18 so normal launches would be tolerable.
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Post #13 by willowbilly3 » Mon Mar 30, 2009 5:35 am

In 1981 I test drove an F100 with a 300 and some sort of OD, probably that toploader style 3 + OD. I am not sure of the final ratio but I do remember I shifted into OD at 65 and put the pedal to the floor. 2 1/2 miles of flat ground later it was still doing 65. That was the biggest pig I ever drove.
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Post #14 by tj300 » Mon Mar 30, 2009 7:32 am

American Thunder wrote:If you think the 300 is bad with 2.75s and the auto, you should see what a 302 does in that situation. Especially with loud mufflers on it, so the whole neighborhood can hear the poor thing trying to accelerate..
I speak from personal experience!

Adjust the timing and fuel, and that 300 will work a lot better for you,
but a swap to a manual trans sure wouldnt hurt.


Why Ford would put a 117hp 300, (possibly around 83-85 rwhp), with an auto tranny and 2.75 gearing in a truck is something I don't understand. With 2.75 gearing and an auto tranny you'd think the 300 would struggle through intown driving just to pull the weight of the truck. That would make it a gas eater in town. Although on the highway, after adding my mods, the power gains are a different story, and the mileage is about fair. It'll really move once I hit about 45mph.
Unfortuneatly, I have to try to avoid freeways because of the poor handling. First year for twin Eyebeam suspension. I have to buy an aftermarket antisway bar. They're not too costly, which is a good thing.
I did notice the rear pinion seal was leaking. A shop in Columbus, Babbits Bearings, will gut the whole differential, replace all the seals, and reassmble it for $500. I thought that would be a good time to buy a 3.50 gear set and have that installed instead of the 2.75.



I would'nt mind the gearing if it were a manual tranny, or the auto if the gearing was at least 3.50.
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Post #15 by StrangeRanger » Mon Mar 30, 2009 8:29 am

There are a lot of trucks out there with automatics and 2.73 or 2.75 final drives and they all seem to do just fine

Get it running right before you throw any more parts at it. If it is as far out of tune as it seems to be from your other thread, it will still be a pig with the 3.50 gears only it will drink more.

The DP manifold is designed to promote fuel ecomony while still offering a modest performance increase when you get into the secondaries. Right now you have neither economy nor performance. Fix that problem then worry about gearsets. Personally, I wouldn't touch a 3.50 gearset with a 300 and a non-OD tranny. Unless this is almost exclusively a tow pig, you don't need or want to turn the RPM required to make 3.50s viable. 3.00 or 3.25 would probably be a much better proposition -- and there's not enough difference between 2.75 and 3.00 to justify the cost of the change out.

As for the poor handling, go through your front end in detail: bushings, kingpins or balljoints (whichever you have), rod ends, pitman and idler arms, steering box, steering shaft joints, etc. Something is shot and needs replaced. This should be your #1 priority far ahead of any performance-related issues.
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Post #16 by tj300 » Mon Mar 30, 2009 8:50 am

StrangeRanger wrote:Your theoretical speeds in gear (3800 RPM) with 235/75-15 tires are 48/78/115 with a highway RPM of 2150 @ 65 MPH. That's not a package for drag racing or towing but it should give very good highway mileage.

The problem is that if you change the rear gears, the highway RPM increases
2.75 = 2150 @ 65
3.00 = 2350 @ 65
3.25 = 2540 @ 65
3.50 = 2740 @ 65
3.75 = 2930 @ 65

You know your tolerance for RPM better than I do. You pick.

Another option is the wide ratio gearset kit for the C6 from Ford Racing, but it's not exactly cheap, somewhere north of $500 by the time you get all the other standard parts that aren't included in the kit. It changes your ratios from 2.40/1.47/1.00 to 2.72/1.54/1.00. That gives you the same overall reduction in 1st as your present tranny with a 3.11 rear but doesn't affect your highway RPM. That's a more substantial difference than it sounds like.

You still have to resolve your tuning issues before any amount of new parts are going to help.


I'm defineately going to do the tuning. But I'm going to put my 300 build on hold for now until I figure out what I'm going to do with it. Whether I want to trade it off or keep it. I need a rear pinion seal. The guy guts the differential and replaces all the seals and reassmbles it for $500. If I can buy 3.50 diff gears and have him install that instead of the 2.75 then that would be a different story. I'd keep it and later have a shift kit installed.
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Post #17 by tj300 » Mon Mar 30, 2009 8:56 am

StrangeRanger wrote:
There are a lot of trucks out there with automatics and 2.73 or 2.75 final drives and they all seem to do just fine

Get it running right before you throw any more parts at it. If it is as far out of tune as it seems to be from your other thread, it will still be a pig with the 3.50 gears only it will drink more.

The DP manifold is designed to promote fuel ecomony while still offering a modest performance increase when you get into the secondaries. Right now you have neither economy nor performance. Fix that problem then worry about gearsets. Personally, I wouldn't touch a 3.50 gearset with a 300 and a non-OD tranny. Unless this is almost exclusively a tow pig, you don't need or want to turn the RPM required to make 3.50s viable. 3.00 or 3.25 would probably be a much better proposition -- and there's not enough difference between 2.75 and 3.00 to justify the cost of the change out.

As for the poor handling, go through your front end in detail: bushings, kingpins or balljoints (whichever you have), rod ends, pitman and idler arms, steering box, steering shaft joints, etc. Something is shot and needs replaced. This should be your #1 priority far ahead of any performance-related issues.


Sorry SR, I didn't see this post. I must've been posting my response as you were posting this. I'm reading it again.
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Post #18 by Lazy JW » Mon Mar 30, 2009 10:43 am

Typical automotive engineering practice for the past 50 years has been to use taller (lower numeric) gearing in vehicles equipped with automatic transmissions due to the effective ratio reduction of the torque converter; If this combination were so very bad they wouldn't have been using it as the predominant design for so long. As StrangeRanger already pointed out, your effective launch ratio is somewhere around 14:1 which is actually pretty low.

My 2.73/T-18 combo at 8.44:1 in second is a bit tall for a 4000 lb vehicle but certainly is manageable with careful clutch work.

The 17.25 overall ratio in granny gear is too low for normal takeoffs on level ground but it is handy if launching under any adverse conditions such as up hill, loaded, etc. It wants to be shifted pretty soon though, and that long shifter throw isn't conducive to great performance.

Throwing money at gearing in search of a magic bullet won't solve your tuning issues. Sorry.
Joe
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Post #19 by tj300 » Mon Mar 30, 2009 11:06 am

Lazy JW wrote:Typical automotive engineering practice for the past 50 years has been to use taller (lower numeric) gearing in vehicles equipped with automatic transmissions due to the effective ratio reduction of the torque converter; If this combination were so very bad they wouldn't have been using it as the predominant design for so long.
As StrangeRanger already pointed out, your effective launch ratio is somewhere around 14:1 which is actually pretty low.


My 2.73/T-18 combo at 8.44:1 in second is a bit tall for a 4000 lb vehicle but certainly is manageable with careful clutch work.

The 17.25 overall ratio in granny gear is too low for normal takeoffs on level ground but it is handy if launching under any adverse conditions such as up hill, loaded, etc. It wants to be shifted pretty soon though, and that long shifter throw isn't conducive to great performance.

Throwing money at gearing in search of a magic bullet won't solve your tuning issues. Sorry.
Joe
I dunno. You guys are probably right. Maybe I'm making more of this than I should.
I should at least be happy that the drivetrain is in near excellent condition with low miles. I'll see what the tuning does.
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Post #20 by willowbilly3 » Mon Mar 30, 2009 2:56 pm

Personally I think the 3.50 ratio on 235s is about right if you expect it to behave like a truck. I agree on getting the loose steering sorted out. Sway bars are fine but they will make the truck ride a lot rougher. I have driven and owned a lot of 60s-70s Ford pickups, mostly F250s and I would never put sway bars on one of mine. But then my truycks are used for being trucks, and your intended use may be different.
Just running tall gears is no guarantee of better milage. I would bet most trucks wouldn't see much change from 2.75s to 3.50s. It still takes the same amount of power to go 65 and the taller gears will just make it work harder to maintain the speed. Actually it would be interesting to see the vacuum gauge readings on those 2.
Those doggy high geared trucks they were pushing in the early 80s didn't get any better milage than the late 70s ones with no overdrive and 3,50 gears.
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Post #21 by StrangeRanger » Mon Mar 30, 2009 3:18 pm

2.75 gears put you at or very near the torque peak of a 300 at highway speeds. That tends to maximize fuel economy. 3.50s are going to raise the RPM by 27% or roughly 600 RPM @ 65. Thaat puts you at a lesser value on the torque, volumetric efficiency and BSFC curves. Less efficiency requires more fuel for a given load.

Basically 3 gears just aren't enough.
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Post #22 by 80broncoman » Mon Mar 30, 2009 5:36 pm

I agree with the above.

If this is only going to be truck in looks only go with a T-5 stang trans. and some 3:50 gears.

If you use it as a truck I'd find a T-18, or NP435 4 speed.

If you want to do niether, get an extra head some SBC 194/160valves and a 4 bbl intake and EFI manifolds. It will totally wake up a 300.
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Post #23 by willowbilly3 » Mon Mar 30, 2009 5:37 pm

StrangeRanger wrote:2.75 gears put you at or very near the torque peak of a 300 at highway speeds. That tends to maximize fuel economy. 3.50s are going to raise the RPM by 27% or roughly 600 RPM @ 65. Thaat puts you at a lesser value on the torque, volumetric efficiency and BSFC curves. Less efficiency requires more fuel for a given load.

Basically 3 gears just aren't enough.


So true but you are dealing with the torque at the rear wheel too and how much more the throttle plates have to be open on any incline. So if 27% is the number the rpms raise then also wouldn't the engine have 27% more leverage over the tire with the lower gear?
If maximum torque was established with wide open throttle plates does that mean the engine is most efficient with wide open throttle plates...well, I am no engineer, just an old ranch boy who knows what a slug drives like and 2.75 geared pickup truck with a 300 would be useless in my world unless someone comes up with a 10 speed transmission. I'm sure it is just right for other people
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Post #24 by StrangeRanger » Mon Mar 30, 2009 6:28 pm

You need 3.50 gears with a V8 because you generally have to rev the pee out of them to get them into their torque band. With a 300, you're there practically off idle. More gear will obviously help due to the greater torque multiplication but is it so necessary that you want to turn over 2700 RPM on the highway? With an OD tranny, I'd opt for 3.50 or even 3.75 gears in a (pardon the expression) heartbeat. I have 3.73s in my 96 with an E4OD but with a C6, I think I'd let the torque converter do more of the work. 2.75 is a little too tall for a lot of uses but it's nowhere near as bad as the 2.47s that Ford put behind a lot of 300s
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Post #25 by tj300 » Mon Mar 30, 2009 6:37 pm

Strange Ranger & Willowbilly, looks like I'll be flipping a quarter. :? :lol:
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Post #26 by StrangeRanger » Mon Mar 30, 2009 6:48 pm

If you're up for changing gears go ahead and do it but take a careful look at those highway RPM numbers and decide what you want. If you had a 3.00 or 3.25 rear there'd be little or no reason to change and 3.50 may be pushing it just a bit too far with a 300.

At the risk of repeating myself: get it running properly and then decide how much more performance you really need
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Post #27 by tj300 » Mon Mar 30, 2009 7:23 pm

StrangeRanger wrote:If you're up for changing gears go ahead and do it but take a careful look at those highway RPM numbers and decide what you want. If you had a 3.00 or 3.25 rear there'd be little or no reason to change and 3.50 may be pushing it just a bit too far with a 300.

At the risk of repeating myself: get it running properly and then decide how much more performance you really need


Oh Yeah, I'll be tuning it before I do anything. Ive been studying the responses on the other thread about how to do the tuning. I want to get my tuning issues straightened out before I decide what to do about the gearing. I just spoke with "Babbits Bearing" in Columbus. He can switch everything over to 3.50 for $572 new seals and all. He said the same as you, that a 3.00 or 3.25 would make little change. He also advised not to go over 3.50. But, I want to see where my tuning puts me.
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Post #28 by willowbilly3 » Mon Mar 30, 2009 8:28 pm

I drove a 72 D250 for 16 years, maybe not the perfect set up but it really worked very well for me. It was a T18 with 3.73 gears and avout a 31 inch tire. that truck did everything I ever ask of it and managed about 16 mpg average. But as I mentioned, any truck I own needs to behave like a truck, I don't use them for commuter car. I realize it is a whole different world than I have experience with to drive around in an F100 never hauling or towing anything.
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Post #29 by StrangeRanger » Mon Mar 30, 2009 8:37 pm

But with what engine?
The RPM at which the engine makes peak torque is essential to picking the right gears.
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Post #30 by Lazy JW » Mon Mar 30, 2009 9:42 pm

willowbilly3 wrote:.....
If maximum torque was established with wide open throttle plates does that mean the engine is most efficient with wide open throttle plates...well, I am no engineer, just an old ranch boy who knows what a slug drives like and 2.75 geared pickup truck with a 300 would be useless in my world ...


Yes, the engine will probably make best BSFC (Brake Specific Fuel Consumption) with WOT at or near torque peak. Please be clear, that is NOT how one is likely to get the most miles per gallon, however.

My White Ox (74 F-350 in my signature) gets to run closer to this condition than most vehicles when I haul hay, and when I calculated the "Ton-Miles per gallon" it is WAY better than my 81 F-150 gets hauling just me and my lunch box to work :D This means that the BSFC on the White Ox is much better when it is working hard than my 81 gets just loafing along.

That said, my 81 F-150 with its dreaded 2.73 gears and the T-18 really isn't all that bad in the hay field. It is certainly leaps and bounds better than the 1989 GMC half-ton I had that was equipped with the 4.3 V6 and the non-granny geared 5 speed. THAT miserable contraption was pretty much un-useable in the hay field, whereas my 81 F-150 in granny gear can just lug along with a ton of hay on it and I don't need to run to keep up with it. It is NOT as good as the White Ox which has 4.10's though.
Joe
"The White OX" 1974 F-350 300-6, Stock single exhaust, Carter YF, T-18A, Dana70 w/4.11, Flatbed dually w/dump bed. "Where no oxen are, the crib is clean, but much increase is by the strength of the ox" (Proverbs 14:4)
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tj300
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Location: Grove City,Ohio,near Columbus

Post #31 by tj300 » Tue Mar 31, 2009 8:03 am

From time to time I'll be hauling 3-4 squares of roofing shingles, along with tools and other misc. roofing materials. I can't haul as much in my F100 as I could in my F250, which I just recently sold.
But to be honest, the gearing I have now had no problem hauling that load I just mentioned.

Also to be honest, I would like the 3.50 gearing for some light to light street performance.

But performance or not I have to have a shft kit. Too much lag between shifts.

I'll wait to see where my tuning puts me.

If I'm understanding correctly 2.75 is going to give me more freeway performance? I.E.- 2.75 - 2150rpm @ 65mph. as opposed to 3.50 - 2740rpm @ 65mph. And the 300 peters out at 3500rpm. If this is correct I'll have to take this into consideration also. But Willowbilly did make a good point about actualluy driving them to really know what you're going to want. Until after my tuning is done I'm leaning toward the 3.50, but also keeping open the consideration of freeway perfomance which is a good point, and the difference between 2150rpm/2740rpm is a pretty good loss of performance. But, I'll see after the tuning.
America was founded by people who believe that God was their rock of safety.I recognize we must be cautious in claiming that God is on our side,but I think it's all right to keep asking if we're on His side;Ronald Reagan, 2nd inaugural

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