Look for a label in the door jamb or on the door. It will have the VIN and engine, transmission and axle codes. Post a,picture of it if you can.

 

Thanks please see att

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you could swap in a taller gear rear differential ,it looks like you have a 2 speed rear end with a 6.59/9.07 rear gears .you need to know your axle diameter and spline count on the end of the axle ,to swap in the correct Meritor differential with a more hwy friendly gear ratio .

 

Thank you so much. This is well outside my wheelhouse, so I definitely appreciate the input. That makes complete sense now. I am very unfamiliar with these trucks, and that helps a ton

 

1FDNF70J8PVA17756.

1FD = Ford Motor Co USA, incomplete vehicle

N = Class 6--19,501-26,000 lbs GVWR, hydraulic brakes

F70 = F700 gas

J = 429 gas V8. (I don't have complete info--EFI or 4V carb--you'll figure it out with a quick glance)

8 = Ignore, Ford check digit

P = 1993 model year

V = Kentucky Truck Plant

A17756 = Serial number

ZA = Diamond white

153 = 153 inch wheelbase

F702 = F700 gas, 24,500 lbs GVWR

8 = This is usually a 3 character code but no upholstery info is given, only body type. This would be standard cab

CM = Eaton model FS-6025-B five speed direct transmission

No axle codes, which is really what we were looking for. This truck was a special order, so the interior and front and rear axle were probably part of that.

87 = Body Company, which would explain in part the special order

0382 = Special Order Number

The axle you have lists itself as Meritor, but I think those were previously Rockwell. The MPC lists a 17220 with a 17,500 lb capacity and 6.50 / 9.07 ratios. Since it seems to be a regular production item, I don't know why it isn't listed.

There are other axles for diesels with taller gears.

What tire is on this truck?

Does the two speed shift into high range?

If this truck has 11R-22.5 tires, those are about 42 inches in diameter.

With that size tire, your direct 5th and the 6.50 high ratio, you should turn 3380 RPM at 65 MPH. The 429 was rated at 235 HP @ 4000 RPM in trucks so it should pull 65 MPH without too much of a complaint.

If you are stuck in Low on the axle at 9.07 you would be at 3990 at 55 MPH and that would be about it as the governed speed would be not much over 4000 RPM.

If you are unfamiliar with these, what is your intended use?

Two speed info here--you have a Rockwell or Meritor axle, but the shifting is the same as an Eaton for the most part. Your axle may be more forgiving but even if it is, never shift it on a down hill. Be in low and stay there.

https://www.ford-trucks.com/forums/1...ml#post9565523

 

You guys are amazing. I’ve probably spent 40 hours trying to figure this out, to no avail.

My truck does have 11r 22.5 tires. It also will shift to both high and low. However, in high gear / 5th on the transmission it hits 4K on the tack at 50mph. In low gear it will go about 35 +/-

 

Is the tach an OEM or aftermarket? If aftermarket, check to see if it is on the 8 cylinder setting.

Was this truck converted to 4 wheel drive? Is there another transmission or anything between the 5 speed and the axle?


50 at 4,000 with a 42 inch tire would require a 10.0 high range.

35 at 4,000 would require a 14.3 low ratio!

 

It appears to be the original tach, but I have questioned it’s accuracy. When it is showing 4K, the motor doesn’t seem to be humming quite that fast.

The truck is 2wd and there’s nothing but driveshaft between the transmission and rear end. Is there an easy way to check the accuracy of the tach? Just based on feel, it seems like it may be 20% too high.

 

A known good tach or a tachometer testing device--you may be able to borrow one from an autoparts store.

There are ways to use a multimeter to check them too--but I'd rather find a test tach that shows RPM.

Do a search and you'll see a dizzying array of devices that can test one.

So first, I would ask at a parts store that offers tools and see what you come up with.

 

Thank you so much. I’ll check that out tomorrow.

 

One more thing… I also have a 78 model f800 with a Spicer 5252-A transmission. It is a 5 speed, but I’m trying to figure out if it has an overdrive 5th gear. Does anyone have any info on that transmission?

Im thinking I am going to make the differential gear ratio more Highway friendly (maybe 3.x high gear and 6.x in low gear. I may also switch out the transmission if I am still getting honked at on the two lane roads.

Thank you all again for your help and time. I have an excavation / pond construction company in Oklahoma. If anyone is in my area I would be happy to offer a steep discount on a new pond to help repay the hospitality on here.

 

That transmission is direct.

I think you should solve the RPM/MPH mystery first.

You may have a tough time trying to pull a 3.something gear.

These trucks are usually pretty close to RPM/HP limited--in other words, even with a taller gear you can't go much faster due to drag and rolling resistance.

 

you can go to the spicer website for the 5000 series to find the spicer 5 speed transmissions with OD ,5753-5853abc do ,but I can't find any for sale .a rear diff swap is your best bet for lower rpms

 

How does he know he needs lower rpms?

Read through the thread--his tach must be way off.