2.94 axle gears? not from factory right?

[1970excursion]

Bronco does not have a lot of getup and go from a stop but comes to life on highway.

no tags on axles so when I wanted to check what ratio I had, I jacked up one tire in rear, put tranny in neutral and spun the drive shaft and watched how many times the tire turned, and it was not even a 3 to 1 ratio. matched on the front, so without pulling the cover and counting teeth on ring gear I am stuck. Would it have come from the factor with a 2.94 gear ratio?

wierd. I will run up to speed and check my RPMS at 60 now that i have a tach installed. also guage reads 85 when gps says I am at 65, does this confirm the ratio or put it in the other direction?

33" tires and a C-4

 

[Broncobowsher]

Completely possible.
Rear end went bad, someone went to a junkyard and picked up whatever was around (out of a car?) and dropped the center section in.

What ratio is in the front? I bet it is different.

 

[rwill]

The tire diameter is what is causing the speedo problem. Ican't imagine the 2.94's being factory ,unless ordered that way.

 

[1970excursion]

Alright i am at a loss,

the front matches, less than 3 turns of the driveline to get one rotation out of the tires.

But,

crusing down the road at 60 (by the GPS) I am right around 3000rpm,
with 33's the calculators say 4.56 gears. Unless my 302 is really weak, or built for top end only, It does not feel like I have 4.56 gears.

I will have to pull the cover off of the front and count each tooth, I am going to crap myself if I just miscounted the revolutions of the driveline.

 

[bax]

Open differentials will fool you. Mark the drive shaft and push the truck forward for one tire rotation. Have someone count the drive shaft rotations.

 

[thegreatjustino]

Open differentials will fool you. Mark the drive shaft and push the truck forward for one tire rotation. Have someone count the drive shaft rotations.

X2 on the open diffs. Use bax's suggestion or get a buddy to help you. Jack up the rear end, one of you on each tire spinning them the same speed. Count the number of driveshaft revolutions for one rotation of the tires.

 

[broncnaz]

X2 on your open diffs causing a issue with your calculations.
2.94 is possible but I doubt it was ordered like that your vin tag should have a axle code to tell you what it should have had.

If you have a open diff Jack up only one tire and turn it 2 times count the number of driveshaft turns that is your gear ratio.

Also keep in mind that 33in tires are almost never 33in tall most are 32.5 or less when new. Also automatic trannys tend to slip so RPM's can be more than the calculaters say. If you have 4.11 or 4.56 gearing and no lowend then something is very wrong either the tranny/ torque converter are slipping a bunch or as you said you engine is built for high end. Does your engine have a good lope at idle? maybe the cam is to big for the converter. Big cams and stock converts dont mix well you'll feel as if you dont have any bottom end.

I run 33's with 3.50 gears and a manual tranny I've got very good lowend with my setup. A lot of people run 33's with 3.50 gears and C-4's with good results as well but lowend can does tend to suffer slightly.

 

[DirtDonk]

Yeah, the 2.94 could have been a factory gear from a large car like an LTD or T-bird. I picked one up with a similar ratio once. Had a pinion-snubber shield and a very long u-joint yoke for a larger series joint and the guy said it had come out of a '70's LTD/Torino or something.

That said, you'll have to do what the others said about having someone help you. Either that or lift both tires off the ground and when you spin it by yourself, make sure the other tire is moving equally in the opposite direction and count the turns. Then double the number you come up with.
Your 2.94 wouldn't compute then though, because that would net you a 5.88 ratio. A 5.83 is available, but either way your rpm info doesn't match either a 2.94 or a 5.88 (or 5.83 as the case may be) so I'd compare your next findings to the rpm/gear ratio charts found here.

Funny though. With your speedo reading about 24% HIGH, either someone replaced the speedo gear or you do have a taller ratio than before. Just changing to larger tires will make a speedo read LOW.
Sounds like it's time to re-test the rotations, and also go in and see how many teeth on the speedo's driven gear.

Since your rpm readings don't equate to either of those ratios, I'd figure it that way.
3000 rpm at 60 mph with 33's sounds like a low ratio to me. Say, 4.11 minimum? Maybe even 4.56's? The C4 will have a couple of hundred rpm extra for slippage, but the rest is just guessing on my part until a gear calculator is consulted.
I would think that 2.94's would have you more down in the 2000 rpm range at least.

As was said, it's possible that your lack of get-up-n-go is a result of issues other than gearing.

Paul

 

[1970excursion]

Spun wheels again. Almost exactly 2.25 revs of the driveline. So that would be 4.56. The speedo gear has 17 teeth, i have a spare 18 tooth but I still dont think that would fix anything right? maybe a 5% change to the better?

I need a 21-22 tooth speedo gear.

 

[DirtDonk]

The 4.56 ratio was actually available in your year I think, so that could even be a factory setup.
Is there a tag anywhere on the front or rear axle assemblies that might confirm this?
And if this is a new-to-you Bronco, it wouldn't hurt for you to verify that the fronts are the same. With that end at least, you can simply pull the cover off and count the teeth on the two gears.
And it serves to change the gear lube too, which is seldom done by PO's it seems.
Diff lube, t-case lube, tranny lube, brake fluid, front wheel bearing servicing, and rear wheel bearing replacement seem to be permanently on the "ignore and it'll be fine" list.

Paul