not sure where to really start but here it is. i have a 94 ford 351w e4od 4'' skyjacker lift and 285/75 pro comp xterrains. my problem is whe i'm going down the highway at 75 the engine is only turning 2000 rpm in overdrive. that's with my foot all the way to the floor. the tranny keeps going from overdrive to 3rd and back nonstop. the tranny has just been rebuilt maybe 1200 miles on it. all heavy duty clutches 6 pinion steel planetarys and a rv/towing conerter.

my questions are why such low rpm at high mph and why do i have to have the thing floored to keep 75. and why does it keep going back and forth from overdrive to 3rd

i had a buddy tell me that the big tires i put on it dropped my rear end ratio from a 3.55 to a 3.00 or around there

i've been thinking about putting a set of 4.11's in but i don't know if that will mess up the tranny or how it shifts as i know nothing about this computer controlled sh*t

all input would be greatly appreciated

by the way the rear end is a 8.8 with 3.55 i think

 

By going with the larger tires you have effectually reduced the gear ratio by 20% +/-. To get close to having the same power before the tires you'll need to step up to 4.56 gears or close to it. If you want more power, a set of 4.88s will give you a tad more. I can only assume driving it now is like driving a land slug.

 

It's probable that the tranny is hunting between gears because of the oversized tires. Have you re-programmed the speedometer to match them? The PCM uses the vehicle speed signal as part of the tranny control algorithm.

In the interim, turn the OD off by pressing the OD off button on the gear selector lever

 

if i go with 4.11 will i have to change the carrier also

 

The 8.8" Ford rear will use the same carrier; the front is different. The Dana 44 uses a thick carrier flange/thin ring rear combination up to 3.54:1. Above 3.54: it uses a thin carrier flange/thick ring gear.

Have you re-programmed your speedo? If you haven't, you need to do that before you throw any more parts and money at the truck. It may solve many of your problems.

 

i have not reprogramed the speedo. i will try that first if you can tell me how. thankyou for all the advice

 

I hope your tires are 285/75-16.
I couldn't find squat on 285/75-15 tires so I assume they don't exist.

Instructions straight out of the Ford Factory Service Manual:

PINPOINT TEST F: SPEEDOMETER/ODOMETER READS INACCURATELY

F4 VERIFY CONVERSION CONSTANT AND DISPLAY TYPE

Check tire size and type or vehicle rear axle.

-- Refer to «Conversion Constants Chart» under Specifications in this section and note conversion constant.

Check whether the conversion constant in the speedometer is correct with the following procedure:

-- Press and hold the RESET button on the front of the speedometer while turning the key to RUN.

-- Release the RESET button. The pointer will prove out and some codes will appear on the odometer display. If an E is on the left side of the display, the module is programmed for English display mode and should have English graphics (mph). A lower case "o" signifies an overseas (metric) graphics (kph). Verify the graphics and display modes agree. (The number appearing after the type is the microprocessor revision level and is not used in this test.)

-- Press and release the RESET button again. The conversion constant and the word CAL will be shown on the odometer display.

-- Verify the programmed conversion constant is the same as that displayed on the chart. Refer to «Conversion Constants Charts» under Specifications in this section.

-- Key off.

Is the programmed conversion constant the same as that displayed on the chart and does the programmed display mode match the speedometer graphics?

Yes
REPLACE instrument cluster as described in this section. VERIFY proper operation.

No
If conversion constant is wrong and programmed display mode OK, RESET conversion constant. REFER to Reset Conversion Constant in «F5».
If conversion constant is OK and programmed display mode wrong, REPLACE instrument cluster as described in this section. VERIFY proper operation.

F5 RESET CONVERSION CONSTANT

Note: The module can only be reprogrammed six (6) times.

Reprogram speed conversion constant using the following procedure:

-- Locate the dealer mode programming enable single-terminal connector. The connector is located at the bottom of the instrument panel below the center of the glove box. On all vehicles, the dealer mode enable wire is Circuit 567 (LB/Y).

-- Insert metal clip in service wire panel connector. Clip other end to vehicle ground while the key is in the OFF position.

-- Key to RUN while holding down the trip odometer RESET button on speedometer. Release RESET button.

-- Speedometer will prove out (sweep across dial and back). The English/Metric and revision levels will be displayed. The last number of the display is the dealer mode programming lockout count. This count shows the number of allowable conversion constant changes remaining. When the count is zero (0), no additional changes can be made to this instrument cluster. If a conversion constant change is required, a service instrument cluster must be ordered.

-- Press and release RESET button again and PSOM conversion constant (without the decimal point) followed by the word CAL will be displayed.

-- Press and release the odometer SELECT button as many times as necessary to change conversion constant to the correct value.

Note: Pressing and releasing the RESET button to lock in the new conversion constant reduces the number of times the PSOM can be reprogrammed by one count. The module can only be reprogrammed six times. If you change your mind and want to go back to the old conversion constant, turn the key off before you lock in the new constant.

-- When the correct new value is displayed in odometer window, press and release the RESET button to lock in the new conversion constant.

-- Key off. Remove the ground wire. Verify proper speedometer operation.

Is conversion constant now set to the desired value?

Yes
Procedure complete.

No
GO to «F4».

Now, what the manual doesn't tell you is how to figure the constant. ProComp doesn't show revs/mile for their tires but BFG and Goodyear both show 285/75-16 tires rolling at 628 revs/mile. Yours should be similar. The constant for 8.8" rear ends is:

revs/mile x .0135 = 628 x .0135 = 8.48

For Sterling rears it is .015 times revs/ mile or 9.42 for your tires.

Hope this helps

 

thankyou for all the help you're a smart man whe it comes to this computer controlled sh*t. i will give this a try. i still am going to swap the gears out because i am going to be pulling a lot with this truck. thanks for all your help

 

This caption also contains a LOT of info about the speedometer, including how to calculate the constant for any tire:

 

When you get bigger tires, you need bigger gears!! I have an 84 Bronco that came with 3.50 from the factory with a 28 in tire. It has 40 in tires now and 4.56 gears. This isn't the optimum gear, but it works due to the granny gear. You need to match the gear ratio with the taller tire so that the truck thinks it still has stock size tires, no matter how big they are. New gears will regain any lost power and some fuel mileage. I live about an hour and 45 minutes from Nags Head. I can fill up on 25 gallons of fuel, go to Nags Head and only burn about 7 gallons or so getting there.

 

I am sure I can rebuild the rear diff myself but does anyone know what it takes to get the front done. looks like kind of a hard project with that IFS

 

The front is easier because of the removeable carrier (3rd member, "chunk"). Getting the chunk out takes some effort, but it's very doable if you're mechanically inclined. And it doesn't actually require as much work as a lot of people claim. If you open the slip joint boot, you can leave the R axleshaft in and worry about the E-ring later. I always replace the E-ring with a fabricated spring ring, like most CV axles have. It makes future work even easier.

To drop the chunk, disassemble the L hub & spindle, then pull the L axle assy. out thru the steering knuckle. Either open the R slip joint boot, or fish the E-ring loose thru the fill plug. Then unbolt the d'shaft & chunk. Have a drain pan ready to catch the oil. It takes some effort to break the sealant & release the chunk from the axle housing, so leave the top 2 bolts in, but loose, so it doesn't fall. For more info, read these captions:

. .

Dropping the whole beam w/diff out isn't much harder.



But it's even quicker to snag another used chunk, rebuild it, and THEN swap out your old one. Less down-time.