what is the prime RPM

[david85]

I have been very interested in the idea of having 2 top gears as you have apparently done, one for towing and the other for running empty. My tranny is a ZF5 and the overdrive ratio is .77 Do you think the 3.08 is too tall for this setup? I am running the same tires you are.

About the 3.08 gears....

If you don't need to tow some 10000lbs consistently, then I would go for it. However, you can never go with 4wd because there is no 3.08 or 3.07 ratio for the D50 or D60 front ends. I think the ZF 5 speed has a granny first gear, so getting heavy loads moving even up hill should be fairly easy even with 3.08s. In my case the low stall torque converter does that job for me.

3.08s with an overdrive transmission in these trucks is ideal for speeds of 70 MPH or up. If you don't spend much time there, the upgrade may not work well for you. In 3rd gear at 70 MPH, my truck will pull hard and steady even with a trailer behind me. 4th gear requires a light foot and sometimes picking up speed for hills. You must also keep a close eye on the side mirror to prevent lugging smoke. Once that happens, there is no gain in fuel economy any more.

So far, I'm the only one I know of that is driving a truck like this with 3.08s. I know that 3.55s and a ZF 5 speed have delivered up to 22 MPG in some cases. I'd love to see what a ZF and 3.08s could do. It seems logical enough to me that a truck could have 2 top gears, one for towing and one for cruising empty, but it seems that even now, there are no trucks build this way. Every truck should have the righ gear for the right conditions.

 

[david85]

trail Junky:

The fan will always turn even when its working properly, but do you actually feel the fan let go when driving it. When you first start out, you should be able to hear it and even feel the drag on the motor. After about 5 minutes, it should let go, this is most noticable if you are climbing a hill when it happens. The fan noise stops, and the truck acts as though a load has been lifted off of it. YES, the fan clutch WILL cost you fuel economy if it doesn't release when its supposed to.

You say you have a topper.....that could already cost you 2 MPG. Is this a high rise? Mine isn't but it does on average cost me between 2 and 3 MPH.

I was never able to break 20 MPG with my canopy on.

 

[trj]

Mine is a 2WD and 9,000 is about the most I would tow with it, legally anyway. Gross combined is 14,000 and the truck weighs about 5,200. 4th is direct drive (1:1) and should be ideal for towing on the ZF. I note your engine has been modified and it turbocharged. Mine is not and may dictate a higher ratio (lower speed) gear. I have seen 3.23 gears in some of the first gen trucks but those may be Dana/Spicer axles vs. the coporate axles. This is very interesting and the way to go I believe. Like having your cake and eating it too! Thanks for the info. Any more thoughts?

 

[david85]

Mine is a 2WD and 9,000 is about the most I would tow with it, legally anyway. Gross combined is 14,000 and the truck weighs about 5,200. 4th is direct drive (1:1) and should be ideal for towing on the ZF. I note your engine has been modified and it turbocharged. Mine is not and may dictate a higher ratio (lower speed) gear. I have seen 3.23 gears in some of the first gen trucks but those may be Dana/Spicer axles vs. the coporate axles. This is very interesting and the way to go I believe. Like having your cake and eating it too! Thanks for the info. Any more thoughts?

You are right about being a modified engine, but my truck is also 6000lbs exactly when its emply compared to your 5200lbs, so you should be able to cruise nicely with 3.08s or 3.23s (I think 3.33 might also be an option).

The truth is when I'm cruising at 70 MPH, I only have access to about 2 PSI of boost because the revs are so low, so even a N/A engine might be able to cope if there isn't much wind or steep hills.

The only real draw back could be starting off the line, but once you are moving all you are doing is shifting later relative to your speed.

 

[Clydesdale]

I have been very interested in the idea of having 2 top gears as you have apparently done, one for towing and the other for running empty. My tranny is a ZF5 and the overdrive ratio is .77 Do you think the 3.08 is too tall for this setup? I am running the same tires you are.

I would think the 6 speed would be ideal there. better gear split, 5th is 1 tah 1, and then 6th is overdrive.

 

[trail junkie]

trail Junky:

The fan will always turn even when its working properly, but do you actually feel the fan let go when driving it. When you first start out, you should be able to hear it and even feel the drag on the motor. After about 5 minutes, it should let go, this is most noticable if you are climbing a hill when it happens. The fan noise stops, and the truck acts as though a load has been lifted off of it. YES, the fan clutch WILL cost you fuel economy if it doesn't release when its supposed to.

You say you have a topper.....that could already cost you 2 MPG. Is this a high rise? Mine isn't but it does on average cost me between 2 and 3 MPH.

I was never able to break 20 MPG with my canopy on.

I never noticed the fan go off, but I do know that when the engine is shut off you can hear the fan "free spin" which made me think it was going bad. As for the topper, I thought it would help not hinder my MPG and yes it is a high rise

 

[trj]

I would think the 6 speed would be ideal there. better gear split, 5th is 1 tah 1, and then 6th is overdrive.

The only problem with a 6 speed is the cost. Especially since I already have the 5 speed in it.....but yes, that would be ideal. I wonder what the bottom gear ratio is on the ZF6 compared to the ZF5?

 

[trj]

The overall happy medium cruising RPM is about 2000. This has enough power to spare to for some extra load, but also is low enough RPM to be fairly economical.

David,
Is the 2000 for a turbo charged or NA IDI? I was thinking the "peak torque" rpm would be the place to be for best fuel economy for whatever speed you intend to curse at, but maybe lower would be better in an empty truck? I like to cruse at about 75 - 80 when I can in light traffic....don't want to see any smoke back there though.

 

[93turbo_animal]

The only problem with a 6 speed is the cost. Especially since I already have the 5 speed in it.....but yes, that would be ideal. I wonder what the bottom gear ratio is on the ZF6 compared to the ZF5?

I'm pretty sure 1st and 5th is the same as 1st and 6th but the gears in between are closer to help with heavy loads not sucjh a big RPM drop

 

[david85]

I never noticed the fan go off, but I do know that when the engine is shut off you can hear the fan "free spin" which made me think it was going bad. As for the topper, I thought it would help not hinder my MPG and yes it is a high rise

Free spinning of the fan at shut off is in itself normal, if the engine is warm. Another way to check is to pop the hood of the truck when its cold and see how stiff the fan is on the water pump shaft, then do the same test when the engine is at operating temperature. The fan should feel very loose on the pump shaft.

The topper alone is a problem though. I actually take mine off in the dry summer months.

The taller the flat surface at the rear of the truck, the bigger the vortex you are dragging with you. Its a similar effect like the air brakes that fighter jets have on the spine of the air craft. The higher the rise, the worse the drag.

What normally happens with an empty pickup, is there is a vortex that settles in the box of the truck thats held in by the tail gate and slowly rotate in the oppisite direction as the wheels of the truck (pass some one else while driving in the rain, and you will see this). This low pressure "bubble" helps the air stream on the top of the cab pass over the box and hopefully drop right behind the tailgate near the bumper. It may not look that way, but the bubble that forms in the canopy is actually helping to form a teardrop shape on top of the box of the truck.

I know this looks like crap, but this is a more ideal shape for a topper:

You must be registered for see images attach

The cross section of the rear of the vehicle should be as small as possible. A bed cover would also work better than a conventional topper.

 

[david85]

David,
Is the 2000 for a turbo charged or NA IDI? I was thinking the "peak torque" rpm would be the place to be for best fuel economy for whatever speed you intend to curse at, but maybe lower would be better in an empty truck? I like to cruse at about 75 - 80 when I can in light traffic....don't want to see any smoke back there though.

I think you would have a hard time holding 80 MPH at 2000 RPM if the truck was N/A, you might need more like 2200-2300 (emplty). Peak torque is 1400 RPM, and I've found that the truck has a hard time holding that RPM unless you are at 65 MPH or less. More speed or load and you are going to need more RPM to get the power.

2000 RPM is a compromise RPM for economy while still having RPM/power to spare for light towing or moderate hills. A turbocharged version of the same engine could probably get similar results at 1600 RPM if the turbo is wastegated. My truck can cruise at 1600, but can't really tow anything at that RPM becuase the boost is so low (1-2.5 PSI).

 

[trail junkie]

Free spinning of the fan at shut off is in itself normal, if the engine is warm. Another way to check is to pop the hood of the truck when its cold and see how stiff the fan is on the water pump shaft, then do the same test when the engine is at operating temperature. The fan should feel very loose on the pump shaft.

The topper alone is a problem though. I actually take mine off in the dry summer months.

The taller the flat surface at the rear of the truck, the bigger the vortex you are dragging with you. Its a similar effect like the air brakes that fighter jets have on the spine of the air craft. The higher the rise, the worse the drag.

What normally happens with an empty pickup, is there is a vortex that settles in the box of the truck thats held in by the tail gate and slowly rotate in the oppisite direction as the wheels of the truck (pass some one else while driving in the rain, and you will see this). This low pressure "bubble" helps the air stream on the top of the cab pass over the box and hopefully drop right behind the tailgate near the bumper. It may not look that way, but the bubble that forms in the canopy is actually helping to form a teardrop shape on top of the box of the truck.

I know this looks like crap, but this is a more ideal shape for a topper:

You must be registered for see images attach

The cross section of the rear of the vehicle should be as small as possible. A bed cover would also work better than a conventional topper.

Sweet, I hate that ugly thing, just thought it was helping so I kept it

 

[trj]

ok, Thanks for the info David. Anyone got any gears out there cheap? I have plenty of 4.10 gears here and would like to try something taller.

 

[Clydesdale]

so.... 37's take my7 truck down to 3.54... 39's.... 3.36

4.10 geared ZF turbo truck.

In OD (5th...) that would be 2.83, and 2.70 final drive... gona turn it (hp output) up a little, not much,
will I regret the decision to end up with 3.36 for a daily driver, don't tow much.

Fastest I go will be/is 65-70... not interested in land speed records.

 

[david85]

Something I should add is that while taller tires will reduce your cruising RPM, they will not necessarily gain you MPGs.

The stock 235/85/16 tires might not be may boring compared to the bigger and wider tires, but the height to width ratio gives them a long stride relative to their frontal cross section. It may not seem like much, but wider tires will add wind drag. Raising the overall height of the truck is also detrimental to fuel economy. This is actually the main reason why 4x4s will on average use more fuel. The added weight does play a small role, but its the added height that takes the extra fuel.

If you wanted to go with 3.08s in a sterling 10.25, you could get an after market set for about $250. Mine are motive gear. I've heard that OEM 3.08s may exist, but never actually saw a set for the sterling yet. Also, you must already have no lower than 4.10 gears, so for example if you have 4.11s, that is already a different case and I've been told you can't put 3.08s in there.