1997 4x2 F-150
4.2L / 4R70W
3.08:1 Open Differential
30" Tires

I've been considering replacing my open differential with an Eaton LS unit. If I'm going to do this, I'd like to have it taken care of before the snow comes. But, I also have some concerns.

Speedo Gear:

On a 1997, it's simple enough to determine which speedo gear I'll need and only takes a minute to change it. My question/concern is: Does the speedo gear also tell my transmission when to shift??? Does changing the speedo gear correct for the change in RPMs, so I won't develop a problem with the transmission???

I haven't settled on which gears, just yet. I'm more concerned with having the ability to actually drive in the snow rather than the performance factor. With a change to 3.55: Any exhaust or driveline vibration, from the engine through the differential, will show up around 10mph sooner than it does now. Even sooner with 3.73 or 4.10....

Right now, I'm around 1,550 RPMs at 65 MPH. I tend to drive about 65, most of the time because the transmission can still handle most hills and this is near our speed limits.

At 65 MPH with:
3.55 = 1800 RPMs
3.73 = 1900 RPMs
3.90 = 1990 RPMs
4.10 = 2100 RPMs

For those of you with lower gears: Is the engine still quite happy??? 2100 RPMs, for more than a few minutes, sounds a little high.

Also: there is the cost. It looks like the Eaton unit, Intallation Kit and Ring & Pinion will run around $550.00. Ford charges 3.7 hours labor to replace a Trac-Loc differential. Is this what I should expect... Around $300.00-$400.00 labor for the install???

Thanks!!!

 

AjRagno;

I went from 3.08 to 3.55 a year and a half ago. I have the 4.2 with a 5-speed. The truck is a much happier machine with the 3.55 gears. It is more powerful and gets better mileage because I can cruise in a higher gear a lot of the time, other than at highway speeds obviously. At highway speeds I did not notice a difference in driveability other than better acceleration. I can tow my camper a lot easier. I usually tow at 100 kmh (62 mph) in 4th @ 2750 rpm and the truck loves it. It gets better mileage than when I try to use 5th. Your truck was available with a 3.55 gear so I don't think you need to worry about over revving the engine. Someone else will probably give you some more feedback about the auto trans question that you had but believe me, the 4.2 is a better engine with 3.55 gears. And use Ford Motorsport gears to simplify the installation. Good Luck, GlennMc.

 

I have a 1997 (with 4.6 but that doesn't matter). If you only install the LS unit then you do not need to change anything.
If you go to taller gears then correcting with the speedometer gear will correct the computer and everything will operate as it does now. On later model years, the computer has to be reprogramed to correct the speedometer. On 97 and 98 (and maybe some 99) the correction is mechanical.
Have fun and good luck.

 

Aj

I had gone from a 97 (4.2l, 5 sp, 4x4, reg cab) with 3.08s to a 01 (4.2l, auto, 4x4, reg cab) with 3.55s. I was expecting al ittle more from the deeper gears. The truck drives well, however I wish I could keep it in OD on the turnpike. On a couple steep hills I need to kick it out of OD to maintain 65-70 mph. IMO 3.73 would be the best all around performer with stock or nearly stock tires. When I do my gear swap, I will go with 4.10s, however I plan on running 285 /70 R16 with an agrressive tread.

WLF is right with the speedo gear. The only time that a gear swap will really mess up a transmissions shift points is when the speedo is triggered by a wheel speed sensor. These use realitive wheel speed for the speedometer. In your case the transmission will still shift at the same realtive output shaft speeds, you will just notice them sooner because the wheels won't be turning quite as fast. Now since the speedo is cable driven, it is the only instrument that will be thrown by the gear change ( still measuring the output shaft speed).

 

I have a 97, 5sp with the 3.55 open (axel code 19). I want to move up to the 3.73 LS. My plan right now is to try and find the whole rear end from a junk yard and just swap that out. The axel code i am looking for is H6