I'd like to replace the differential gears in my 1988 F150 with a taller (I assume numerically lower) set, to lower the engine speed on the highway. From my limited research on the web, it appears I can take a set from any 8.8" Ford diff.

Is my information correct? Has anyone done this before? How hard is it?

Thanks in advance!

Pete P.

 

Yes, numerically lower gears will lower the engine speed for a given wheel speed. As a general rule, used gears are anchors - don't reinstall them unless you know EXACTLY what condition they're in and why they were pulled. Considering the high cost of labor to set them up properly, it's just not worth the risk.

It's much easier & more reliable to install a replacement axle as a whole assembly, and it might even cost less overall. You need an 8.8" from any '87-96 F-150 or Bronco with 3.08 gears (AXLE code on the door sticker should be H8 for l/s, or 18 for open diff.)

I recently swapped 3.08s back under my truck that way - see here: http://www.webphotos.com/list_photos...smi=2&a=109397 .

 

What ratio do you have? It might not be worth the money and aggrivation. You might not gain enough to outweigh the costs.

 

Thanks for your help! I don't know the ratio, yet, as I haven't had the opportunity to climb underneath and look. I assume it's very low. FWIW, I have an 88 F150 with a 302 and 4 speed T10 transmission. It's running at 3000 RPM at 70 MPH. Except for the gas mileage, it's a great truck!

 

Do you have the stock size tires on the truck, or something bigger? If they are around the stock size, you've probably got 3.55 gears in there now. Dropping to 3.08's is only going to give you about a 300 or so RPM drop on the freeway. If you're serious about keeping the truck and willing to spend the money, you might want to consider changing the transmission to a 5 speed w/overdrive instead of changing the rear gears. You'd probably see more like a 500 to 600 RPM drop with the OD. Just a thought.

 

I'd say just put some bigger tires on it lol.

 

Right!

Actually, I saw differential gears (new) for about $200. That's even cheaper than new tires.

The transmission swap is just more than I'd want to handle. Then I'd have to deal with things like driveshaft length, etc.

Thanks, all!

 

Well, find out what size gears you have first. Then we can go from there.

 

OK, I've got a 3.55 gears in it. Going to a 3.08 will drop my cruise RPM by about 400, if I did my math right. Is this a real chore?

TIA

 

Some very precise adjustments have to be made to make it work right. I've never done it myself so I can't say what's involved, but I know it's something I wouldn't attempt.

 

Pay a pro you trust. It's a precise procedure.

 

Is it a 4wd truck? If so you must change both sets.