70 MPH at lower RPM

Ray-Ray

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Recently bought an 86 F250 2WD ATS Turbo 6.9 Diesel with 77,000 miles on it (not sure if it's a roll over miles) and I "want" to reach 70 MPH on the freeway with lower RPMs. I was going about 55 MPH at close to 2000 RPM and last time i did try to drive it at 70 MPH the RPM was a little high in my opinion. I'll be towing probably around 5,000 LBS worth of boat and trailer, but not all the time. I am new to FORD and diesel engines and only been reading the threads from this and other diesel sites. any pointers or advise is welcomed, thanks in advance.
 

franklin2

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You didn't say what transmission you have. I would assume it's a 4 speed or a c6, and you most likely have 4.10 gears in the rearend. All you need is a overdrive transmission. It's a wonderful addition and changes the whole personality of the truck. Mine had a c6 with 4.10's. All I could get out of it was 12 mpg. And it was very noisy on the highway. I swapped in a zf 5 speed that has overdrive, and now I get 15 mpg and it's so much quieter and nicer to drive on the interstate. If I am pulling a heavy load I can't really use 5th, you might be able to to if you have smaller tires and a turbo.
 

david85

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If we assume zero slip, the numbers work out roughly to:

2000 RPM@ 55MPH = 36.7 RPM/MPH

So, 70 MPH * 36.7 = 2545.45 RPM

My truck had the a C6/4.10 combo and I can tell you that with that setup, you will have a top speed of 75 MPH. When I realized that, I swapped to 3.08 gears...barely 3 months after buying the truck.

The bad news is, I think you already have 3.55 gears.

Here are a few more numbers for comparison at 2000RPM:

3.08: 61.29 MPH
3.33: 56.69 MPH
3.55: 53.18 MPH
4.10: 45.93 MPH

*This is based on an assumed stock tire diameter of 31.73" (235/85/16 tires)

@70 MPH, your engine RPMs should look like this:

3.08: 2284.10 RPM
3.33: 2570.29 RPM
3.55: 2632.65 RPM
4.10: 3040.56 RPM

If you have the 3-speed C6 Automatic, we can probably add up to 250 RPM to those, depending on load. Expect more when you are below the torque converter stall speed.

You could try hunting for 3.08 gears, but nobody wants to make them anymore. I'm beginning to think I bought the last set.
Another option is adding an auxiliary overdrive, but that gets expensive and will only produce 20% drop in RPM if you have a manual transmission. If you have an auto, the torque converter slip will eat up much of that.

You could go nuts like I did, and swap in an E4OD, but that is also expensive and it doesn't have as good a reputation compared to the transmissions offered in 1986. Mine is rebuilt and lightly modified to be more reliable.

The cheapest and easiest solution would probably be converting to a used ZF5 speed manual transmission. This is what most guys end up doing.
 

Ray-Ray

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thanks for the replies, the truck has a C6 with a gear vendor, i researched the the gearing and its .78 for there GV. i have 265/70/17 for tires. My ID tag is a little blurry but i think the axle code is C5 (limited slip with 4.10) if the prevoius owner didn't change it. i thought about installing an E4OD but since i have the GV it's almost the same, i'll try and look for 3.08 gears and see what i can find. i'm not planning on going too crazy on MODs for this truck, i just want her to last.

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franklin2

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So the gear vendors is why the previous poster calculated your final gear ratio at 3.55. 3.08 is a dramatic drop, I am not sure how it would tow like that. Mine is barely tolerable for towing. What brings me near the edge is I am running 35 inch tires. I have to use 1st gear in the zf to get going from a stop.

I wonder what my "virtual" final gear ratio is with the 35 inch tires. If the 3.08's are going to take you lower than mine you might not like it for hauling and towing. I see you already have oversize tires on it.
 

Sidewinded_idi

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You have a gear vendors with that high of rpm? Are you sure it’s working. My truck with the e4od and 3.55’s with stock tires will be at 2K Rpm’s at 65. 75 maybe 2300. Something’s not right if you have od and still turning that high
 

Hydro-idi

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I bought the last new old stock 3.08 ring & pinion for the sterling a few years back. The differential shop who I had put them in couldn’t believe what he was looking at. Didn’t even know they existed.
Obviously I don’t think the truck will tow 10k all that well, but it has been towed with & it does just fine. But truck also has a new c6 and engine so I bet that makes a big difference compared to a worn out original 6.9 with lots of miles on it
 

Ray-Ray

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The GV works in my opinion, i can feel it shift to OD at about 45MPH. also that's one of my "i think an issue" is it shifts fairly hard coming out of OD. i email GV and still haven't got a reply. what exactly is a "healthy" RPM for these IDI's? i could just be thinking there's an issue when there isn't? i'm only going by what i've read in some other posts )1600-1800 RPM at cruising speed on the freeway. Thanks again in advance.
 

Macrobb

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Yeah, GVs shift /down/ pretty roughly. I have one in my '88 with zf-5, and I always put in the clutch before shifting it up or down. Upshifts can happen 'on the fly', downshifts are rough.

As far as RPMs go, NA, your torque peaks at like 1400RPM and goes down from there. As long as you aren't running fully loaded below that(and make sure you aren't smoking(high EGTs)), you should be fine.

1600 RPM @ 60MPH felt 'good' N/A, but going 75 at 2000 was pushing it; it just didn't have a ton of torque to push the truck. It'd do it on the flat, just not so well up hills.

Once I stuck a turbo on it, it made a *big* difference. 2000 is perfect for 75, with tons of turbo-assisted torque, and I've even hauled a moderate load(probably no less than 5K) in double-overdrive doing that without issues(Note: while you can haul in double OD at 70-75, you have to downshift to single OD when hauling at 60-65 - the RPMs are too low to produce good boost at 1600).

Now, a properly-tuned, in-good-shape NA IDI might change that a bit, but that's just what I experienced. This is also why I make sure that all my IDIs have a turbo now - I'm totally spoiled.
 
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Sidewinded_idi

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The idi can run safely up against the governor all day. In marine applications they have no problem and other members report holding it against the governor with no issues. I’m like you I don’t really like that thought but there’s got to be something else going on as like I said my Truck with 3.55 and od is much lower
 

Ray-Ray

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I'll be opening the pumpkin and see what's installed, could it be a case where the previous owner installed a shorter gear like a 4.73 or even shorter?. i suck at ring and pinion math but i'll do what i can (ring tooth divided by pinion tooth or vise versa). if i understand it correctly, the GV is either in OD and out of OD. now from what i've read here before, you could manually engage it every time the transmission goes up a gear or leave it in automatic operation and it shifts into OD when the pressure or speed is within it's shift point.
 

david85

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The GV works in my opinion, i can feel it shift to OD at about 45MPH. also that's one of my "i think an issue" is it shifts fairly hard coming out of OD. i email GV and still haven't got a reply. what exactly is a "healthy" RPM for these IDI's? i could just be thinking there's an issue when there isn't? i'm only going by what i've read in some other posts )1600-1800 RPM at cruising speed on the freeway. Thanks again in advance.

1600 RPM is not going to be easy to achieve when you run a C6 slushbox. Even if you could get into a range where the final drive calculates out to that, torque converter slip will add a couple hundred RPM. This isn't as noticeable at lower speeds (say, under 45 MPH) when loading on the drivetrain is less. Having said all of that, here's what the numbers look like:

70MPH

4.10 and 0.78 OD = 2371.6 RPM
3.55 and 0.78 OD = 2053.5 RPM
3.08 and 0.78 OD = 1781.6 RPM
3.08 and 0.712 OD = 1626.3 RPM*

55 MPH

4.10 and 0.78 OD = 1863.4 RPM
3.55 and 0.78 OD = 1613.5 RPM
3.08 and 0.78 OD = 1399.8 RPM
3.08 and 0.712 OD = 1277.8 RPM*

*E4OD overdrive ratio with no slip

As you can see, 55 MPH should already have the RPM below 2000 with your current setup, but torque converter slip appears to be eating up some of that, which is why you observed 2000 RPM. Since the truck is 2wd, you could easily swap to 3.55 ratio for a reasonable cost, since there's no front axle to worry about. You'll have to decide for yourself if the 300 RPM drop @70 would be worth the trouble.

To calculate ring & pinion, divide the big gear by the little gear. Example:

40 teeth / 10 teeth = 4.10:1 Axle ratio
39/11 = 3.55 (Calculates to 3.54545454...)
40/13 = 3.08 (Calculates to 3.07692307...)

If your calculated number comes out to less than 1, you got the numbers reversed. Simple.

Nice truck, BTW! I always liked that style of front bumper.
 

Thewespaul

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With the C6 being a non lockup trans, you’re kinda chasing your tail here. Sure you can throw more gear into it and bigger tires but you will be sacrificing a lot of performance and you’re combined city highway mileage may go down because the truck has to work much harder to get up to speed. I would seriously consider getting an overdrive trans in there.
 

franklin2

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Yes, you will never get rid of the slip from the c6. And like was said the more load you put on it, the more it will slip.

2000 rpm gives me the most power when towing a load, and I get that easily in 4th gear going down the highway. It also turns the fan fast enough for cooling. 5th gear for me is just a running around empty grocery getting gear. It's interesting since I locked my fan solid to see the rpm required to get proper cooling through the radiator.
 

Macrobb

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Yes, you will never get rid of the slip from the c6. And like was said the more load you put on it, the more it will slip.

2000 rpm gives me the most power when towing a load, and I get that easily in 4th gear going down the highway. It also turns the fan fast enough for cooling. 5th gear for me is just a running around empty grocery getting gear. It's interesting since I locked my fan solid to see the rpm required to get proper cooling through the radiator.
What rear end? I don't see anything in your signature.
 

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