I have 3.08's and they are nice for gas mileage, however leave something to be desired when towing, My question is, if/when I find another F150 with the 3.55 gears what do i have to do to make the speedomer read correctly. Thank you for the help

 

What year truck do you have? If you have a 92 or newer you can recal the speedo your self electronicaly and if pre 92 then you need to swap out the plastic piece on the end of the speed-o cable that goes in the trans. I for got which way you go with the count on the teeth to make it read slower of faster.

 

Actually, on a 1992 F150, the speed reading on these trucks goes off the Vehicle Speed Sensor on the rear diff. and a tone ring attached to the carrier. (true for 1992-present trucks) Changing gearing doesn't affect speedometer reading at all. The only thing that does is a change in tire diameter. On the older VSS systems from 1989-1991, the sensor is located on the transmision or on the transfer case. In this scenario, a gear change or larger tires will require you to recalibrate the PSOM.

 

you do not need to find a 150 w/3.55. you can order a new set from ford racing or summit.
RTM
how do you recalibrate for tire size? i have 295-50-15. do you need a programmer or no?

 

Here's an article I got from another web page. It works for 1992-1996 F150/250/350 and full size Bronco's:

Recalibrate Electronic Speedo:
To adjust the Speedometer and odometer on the late model Bronco. (Note: The module can only be reprogrammed six times.)

First, get the tire revolution per mile. Then multiply that by 108 and divide by 8000. This end figure is your CONVERSION CONSTANT.
AKA: 136.1347721231/(Rolling radius of tire) = CONVERSION CONSTANT
EX: 16" rolling radius= 8.5084231 CONVERSION CONSTANT

RESET CONVERSION CONSTANT
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 a jumper wire or alligator clip in service wire and clip other end to vehicle ground with the key in the OFF postition.

Turn key to RUN while holding down the trip odometer RESET button on the speedometer. Release RESET button.

The Speedometer will prove out(sweep across dial and back). The English/Metric and revision levels will be displayed. The last number of the display id the dealer mode 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 revision is required a service instrument cluster must be ordered.

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

Press and release the odometer SELECT button as many times as necessary to change the 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. 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 new correct value is displayed in the odometer window, press and release the RESET button to lock in the new conversion constant.

Turn the key OFF and disconnect the ground wire and check to see that the speedo works correctly

 

The reason I was thinking go with a rear from another truck, is pricing them so far, it is considerbly cheaper than having it regeared. I know this is crazy, but true. Thank you so much for the help, I do just want to verfy about the speedomter though, if I change the gears they will be rotating faster to turn the tires once, will that not throw off the speedometer?

 

If you have a 1992 or newer truck, then there will be no speedometer change. If you have 235/75R15 tires on, they will still spin at 728 RPM at 45MPH. Remember that the speed sensor is located on the diff which drives the wheels. The engine, transmission and driveshaft are going to spin at a different rate than before but neither of them has the VSS mounted on it. Don't put a Ford 9" rear on it as they have no tone ring or place to install a VSS. try to stay with a 1992 or newer rear so that the VSS plug has a better chance of matching.

 

I disagree with the speed-o not changing. It will change with any gear ratio change or any tire change. n578md, you said so your self "The engine, transmission and driveshaft are going to spin at a different rate than before" This will affect the speed-o reading because engine rpm and your final tire rotation ratio affect each other.

 

The wheels and carrier/ring gear/tone ring spin at the same speed at 45 mph, period. The speed sensor on a 1992 F150 is NOT on the transmission and NOT on the engine and NOT on the driveshaft. It's on the carrier sandwiched between the ring gear and the carrier flange. 235/75R15 tires will spin 728 times per minute at 45 MPH no matter what gear is installed. You can put a 780:1 ring gear, the tone ring will STILL spin 728 times per minute at 45 MPH on a 235/75R15. The only difference that you will be able to tell is that the ENGINE speed at 45 mph was about 1250 RPM with the 3.08 and now it will be 1550 RPM with the 3.55, the wheel is still going 45 mph/728 RPM. Now if you put a larger tire, say a 255/85R16 it will now take 620 RPM at 45MPH and that will affect the speedometer. Capice?

 

That makes sense as long as the computer doesn't read the engine rpm to determine the speed.

 

Yes, that was a concern on the older non-VSS trucks 1988 and prior where you had the little gear to change out on the side or tailshaft of the transmission.

 

gotcha ya.