I'm going to be getting a 3.08 rear axle tomorrow and wanted to put a limited slip in it seeing how it's presently open (but just rebuilt). I was wondering what the sline count was because I'm not sure if it's 28 or 31.

Will

 

If it's from an F150 it has 31 spline axles. Why would you purposely buy a 3.08 axle though, it's pretty much useless for any truck with an OD trans.

 

My truck is a 5 speed and I do a lot of highway. I currently have 3.55 an I don't like sitting at 2500 on the highway. The most this truck will ever have to pull is 3000lbs so I would think it could do that. Plus I'm getting a pretty much brand new rear end for $200 (just rebuilt and painted).

 

Plus highway traffic is nice when you have a bit longer 1st to just put in instead of always bogging and killing the clutch by leaving it in 2nd. I'm also only on 31's and don't plan on going any bigger.

 

If your axle is the factory truck axle it's 31 spline. I want to second what Conanski added, I've had a truck with factory 3.08 gearing & 31x10.50s and later swapped a set of 3.55 into, I would NEVER EVER go back to a 3.08 setup on anything agian especially if I ever planned to tow/haul even a small load with it.

If its what you really want and you're sure it will be ok for you then by all means go ahead. Just wanted to toss in my 2cents.

Don't see a truck listed on your profile so just to be safe, if you own a 4x4 don't forget you'll also have to regear your front axle or you'll be breaking very expensive stuff in short order.

 

You are going to HATE the 3.08s

 

It's a 1994 ext cab 5.0 5speed 2wd. I just got her running like a champ and will be doing some modest power adders starting next week. I will be doing a comp cam 255, gt 40 heads, long tubes to single exit, 1.7 rockers and will advance timing a bit with 24lb injectors and a good tune. Would it still be a dog with an extra 70hp and similar tq? I'm not looking for a fast truck, just something with a bit of go when I need it and good dd capabilities. Ohh almost that I'm doing a maf conversion now as well.

 

Yes, it will STILL be a dog, even worse with 3.08s

 

Have a look at this chart..
Jeep Parts, Jeep Accessories, Gear Ratio Calculator - OMIX-ADA
You are already on the better fuel milage end, you go to 3.08s you will be in the GUTLESS POS end

 

edit: Damn Brad beat me to it and had the chart I couldnt find when making my post, thanks for the link man have to favorite that so I can find it agian.

Yeah all those upgrades would make it go better with a 3.08 rearend, but here is the problem, with all those addons and going with the 3.08 gears your really shooting yourself in the foot. The power you gain from the mods your going to lose from dropping the ratio. Essentially you will be putting money into the motor to balance the loss of power from a ratio drop. Where with keeping the 3.55 gearing it will be a solid power gain, no loss of any power what-so-ever. That's what we're trying to explain to you.

I did some quick conversions, right now with the info you've given you look to be turning about 2500rpm @ 80mph, with the 3.08 gearing you would drop down to about 2175rpm @ 80mph. It's just really not worth the loss of power you'd take from switching to 3.08s just to drop about 300rpm at cruise speed IMO. Any mileage you might you might gain from dropping 300rpm will be lost by trying to push a full size extended cab truck with 31x10.50s with a 3.08 rearend.

TLDR version: If I were you I would keep it as is and just get a LS for my axle and call it a day.

 

That's what I was about to say, you're driving 80+ mph which is way too fast to get respectable fuel milage, and that's going to go down(not up) with 3.08 because the 5.0 is a gutless wonder that has to rev to make usefull TQ. If you want a slight improvement in milage simply slow down to 70mph, right now you have the perfect trans and gearing combo for that engine, higher gearing is just gonna screw that up.

 

I definitely don't cruise at 80? I don't know were you got that from. I sit at or less than 110km/h and I sit around 2500rpm. the code on the door is 19 which is 3.55 but I understand that the rpm and speed don't match up well because I've done the calculations my self. Whats the next step up from a 3.55 with ford? I'm wondering if the previous owner changed it out. I drive the truck nice and easy seeing how I need to go to work everyday and I don't want it to break. I will take your advice and stay away from the 3.08. I wonder what my rear end is now if it's not the 3.55. My tires are 275 15's which is a 31 so.............

 

Assuming your in 5th with your M5OD, at 68mph(110kph), with 31's and 3.55's your engine rpm should be around 2067 rpm.

(68mph*.79(5th gear ratio)*3.55(axle)*336.13/31(tires in inches))

Assuming 4th:
(68*1*3.55*336.13/31) = 2617 rpm. Right where it should be.

With 3.08's:

5th:
(68*.79*3.08*336.13/31) = 1794 rpm. WAY too low.

4th:
(68*1*3.08*336.13/31) = 2270 rpm. Still too low.

Stick with 3.55, which ARE what came in your truck.

 

The reason I thought you we're doing 80MPH (or 129KPH) is because I looked up the gear rations of your trans, and you said you had 31's. With that info and the ratio in your rear axle you can do the math and tell what speed your doing at what engine RPM. What you need to do is look at the tags on the driver's side door and see what it says your truck's original tire size is.

It sounds to me as your speedo is reading lower than it should, not sure about your truck for sure, but my 92 F150 and my 92 Bronco I had both came from the factory with 235/75r15's. I'd be willing to bet yours came the same way and your speedo is reading incorrectly with the current (larger than stock) tire size you have on the truck now. So while your speedo is showing you doing 110KPH you're actually going 129 +/-KPH. No way with your M5OD & rear axle gearing are you only doing 110KPH at 2500rpms in 5th gear.

 

See my calculations above. Possible if in the right gear. Depends on what gear he cruises in.