Changing the final drive gear in the differential

[sandmann999]

Hey everyone,

I'm thinking about changing my final drive in my differentials. I have the 4 speed Overdrive transmission with the 2.73 final drive. I imagine this set up would be fine for a V8, but with the I6, I just don't have the power to turn over such large gears at high speeds. When I'm at 65mph (which isn't the easiest to get up to sometimes), I'm at 1700 RPMs and have absolutely no power. After doing some calculating, I figured that if I put the 3.73 final drive in, I will be around 2400 RPMs at 65mph, which I would assume would give me a little more cruising power AND better gas milage as I've heard they get the best gas milage around 2300 - 2500 RPMs. Secondly, when I'm off roading, the lower gears will help me out there as well.

From what I've figured, putting in the 3.73 instead of the 2.73 final drive will make a better difference all the way around. Are these assumptions true? Or am I off?

If so, what is entailed in switching the final drives and where can I get them?

Thanks!

 

[Seabronc]

If you did that calculation from Jeff's Bronco Graveyard catalog you are probably right. We can't cross check your calculation since you left out a vital piece of information, the tire size and estimated highway cruising speed. Also calculations should be done at the 1:1 condition not the over drive ratio. You can apply the OD ratio to it after to see what the highway RPM would be.

Good luck,

>-

 

[sandmann999]

If you did that calculation from Jeff's Bronco Graveyard catalog you are probably right.  We can't cross check your calculation since you left out a vital piece of information, the tire size and estimated highway cruising speed. Also calculations should be done at the 1:1 condition not the over drive ratio.  You can apply the OD ratio to it after to see what the highway RPM would be.
Good luck,

>-

33351[/snapback]

​

Ok, here are my current specifications:

1st: 3.29

2nd: 1.84

3rd: 1.00

4th: 0.81

Final: 2.73

Tires: 215 75R15 (it came with 235s, but when I first got it, I didn't know any better and the tire shop guy said his computer said 215s. Grrr....they look so little!) However, I'm hoping to put some 265 75R15s on there, so I'm basing my calculations off of that.

(Hopeful) cruising speed: 65

My tires say they are 30" x 9.5", but they measure a little less than 28" across (which is a lot closer to the calculated 27.7" of the 215 75R15s) However, with 265s, they'll be 30.65" across.

 

[sandmann999]

Is it very difficult to change the final drive? Just curious how to do it.

 

[Seabronc]

Is it very difficult to change the final drive?  Just curious how to do it.

33783[/snapback]

​

Most people would recommend that you leave that to a professionsl. That is because if you don't get it adjusted properly after changing gears they will tear up in no time.

>-

 

[$100.00Bronco]

Rember that your final drive is with overdrive so your axle could be 3.55 or 3.73. Once you know your axel ratio then you can calculate what axle gears to swap in.

Randy ring&pinion as a good gear calculator on there web site.

Dennis

 

[ocalabronco]

if you havent already done so, look on your rear diff. there should be a tag on there to tell you what your ring and pinion is. i cant remember ford ever using that high of a ratio in trucks with a 4 spd auto. probably 3.55 or 3.73, at bare minimum 3.08

 

[sandmann999]

if you havent already done so, look on your rear diff. there should be a tag on there to tell you what your ring and pinion is.  i cant remember ford ever using that high of a ratio in trucks with a 4 spd auto. probably 3.55 or 3.73, at bare minimum 3.08

33850[/snapback]

​

I'll have to take a look at the rear diff., although it's a 4 spd manual, not auto. In the OD in an '81, was the OD gear 0.71:1 or 0.81:1?

 

[ocalabronco]

what is the shift pattern like? that will tell me what manual trans you had. if you DO infact have OD on this trans, its probably the SROD manual trans, because i know NP435's had a 1:1 final gear, and T-19s weren't far off.