Did 3.73 rearend swap.. now thing but more problems

wazazzle · Apr 23, 2006

So I just swapped in a 8.8 w/3.73 I found at the local yard. It came from a 90 5.0 automatic that was already worked on. I went over double checked everything twice after installing and have the correct speedo gear from CJ ponies. Now my rear end is causing nothing but problems. First problem, I'm on the hwy I'm hitting 60mph in 5th gear @ 4500rpms!!!! Which is not good.... I should be about 2000rpm. It's not the tach. I know the diff between 2k and 5k. It is def 5,000rpms! I monitored a minute at 60mph, and I traveled 1mile per min. So, I'm kind of at a loss for words and explanations. I also noticed my driver side wheel wobbling. So I take it to a buddies and we think it's either a bent axle or the housing is bent. But when I had it up on jacks the day before it seemed straight. So I'm leaning towards maybe the axle is bent to ****. That still doesn't explain my rpms??? My clutch isn't slipping... and Yes I did open the rear end up. And yes it was stamped with 3.73. And yes I verified also by comparing rotations... it is a 3.73. Can anyone help... or Any one else experience this same problem???

hllon4whls · Apr 23, 2006

Make sure that you have the right gears. Mark the tire and the yoke. The yoke should spin 3 and 3/4 turns for every 1 turn you rotate the tire. Anything wildly different and you dont have the gears you think you have.

I just dont understand what could possibly be happening. How do you know you are going 60? Have you clocked yourself? Have access to a GPS or someone with a new car and stock size tires?

wazazzle · Apr 23, 2006

hllon4whls said:
Make sure that you have the right gears. Mark the tire and the yoke. The yoke should spin 3 and 3/4 turns for every 1 turn you rotate the tire. Anything wildly different and you dont have the gears you think you have.

I just dont understand what could possibly be happening. How do you know you are going 60? Have you clocked yourself? Have access to a GPS or someone with a new car and stock size tires?

I verified it was a 3.73 rear end cause it was stamped 3.73... and I also turned it and is spun about 3 3/4. So yes it is a 3.73 and I when I was on the hwy my tach was hittin about 4500-5000rpms at about 60-65mph. I was trying to keep up with traffic which was about 70-75mph and my motor was starting to red line in 5th when I tried getting to their speed. I did ask for a 1989 5.0 3.73 speedo gear from CJ Ponies. They sent one and I installed it. Didn't see any 3.73 marking or ID# on the pkg but trust they sent the correct one. Even if they sent an incorrect speedo gear. Why am I able to feel my motor hitting 5k? It was definately NOT 2k! I took it to my buddies place. He's also the ford tech who helped me install it. We were at a lose for words and just dumbfounded.

hllon4whls · Apr 23, 2006

What about a completely broken axle? Did you pull the axles when you installed the new rear?

You can jack the car up and in neutral both axles should spin the same direction (posi) or in opposite directions (open). If you can hold one tire with no resistance the axle might be broken.

If the axle is broken, then the car is not safe to drive.

Thats the only other plausable thing that I can come up with.

Since you have your old rear (I ASSuME) you can either swap back to it and forget the whole thing or you can trouble shoot and attempt to fix.

Fett · Apr 23, 2006

This may be a stupid question....but what engine do you have?

You mention that the rear end was an 8.8, which almost sounds like you didn't have an 8.8 to begin with.

I know it may be a very stupid question....but I thought I would ask anyway. Because the speedo gear wouldn't do anything except make your speedo accurate, and if you know for a fact you were going 60mph, and you know you were at 5k rpms in fifth gear...something is amiss.

blue302stang · Apr 23, 2006

Even if it was the incorrect speedo gear, that has nothing to do with the reading of the rpms. Did you check both the ring and pinion for proper stampings?

hllon4whls · Apr 23, 2006

blue302stang said:
Even if it was the incorrect speedo gear, that has nothing to do with the reading of the rpms. Did you check both the ring and pinion for proper stampings?

After I thought about this for a while, something occured to me. Even with 4.56 gears your RPM's would not be 4500 at 60. I dont think that the 8.8 even goes much higher than that for gear ratio availability.

My bet is still a broken axle or something wrong in the carrier. If the axle is indeed broken, this is an easy fix. ESP if the original rear is still available.

wazazzle · Apr 23, 2006

blue302stang said:
Even if it was the incorrect speedo gear, that has nothing to do with the reading of the rpms. Did you check both the ring and pinion for proper stampings?

Yes it was stamped with the proper marking 3.73.... The mph was correct too because on my way home from noticing the problem. My radar went off and I slowed to 35mph and the cop zapped me. He let me drive pass so I'm assuming I was doing the speed limit.

**Fett** My 89 5.0 had it's stock 8.8 rearend, so it wasn't like it was a 4cyl conversion. Only swapped it for the 3.73 gears.

**hllon4whls** I checked to see if both wheels turned on my posi rear, they do. Would it still be able to do that if the axle is broken? I'm curious cause I'm guessing if it was broken I wouldn't be able to do the correct speed limit. I did check 1 minute at 60 mph,,, and my tach rolled 1 mile.. So I'm pretty sure mph isn't off.

wazazzle · Apr 23, 2006

Either way I'm going to have to pull out and maybe replace that driver side axle if it is broken. I will also prob pull out the passanger side too just to check and be one the safe side. When I replace my "maybe" bent axle with the old ones it might majically solve that rpm prob too. This is a pita but now it's giving me a bit of an excuse to finally swap to 5lug. Or if I'm lucky find a TC rearend this wk at the pullpart. Still interested in hearing anyone elses suggestions though!

wazazzle · Apr 23, 2006

hllon4whls said:
After I thought about this for a while, something occured to me. Even with 4.56 gears your RPM's would not be 4500 at 60. I dont think that the 8.8 even goes much higher than that for gear ratio availability.

My bet is still a broken axle or something wrong in the carrier. If the axle is indeed broken, this is an easy fix. ESP if the original rear is still available.

Yeah... I'm pretty much concluding it might be a bent or broken axle too now. You're right about the 4.56 gears... here's a chart I found and it still just makes me wonder wtf is going on...

rd · Apr 24, 2006

I have a 4.10 in my 82 and a 4.56 that's been in there, and it turns about 3000 rpm at 55 or so, with a C4 and no od.

You definitely have a problem. The speedo gear is the least of the problem.

Sounds like missing splines on the axle or side gear, or something else in there.

86bluecobra · Apr 24, 2006

this maybe a stupid question but could it be worn bearings in the rear end? causing the egine to have to work way harder to get to the spd? it seems very strange to me. since it is a c clip rear end your entire wheel would fall off if there was a broken axle. i doubt it is that. is there any wierd/loud noise form the rear? what gear tooth did you install in the tranny?

95GTAODE · Apr 24, 2006

I'm willing to bet that you swapped a 5.0 into a 4 or 6 cyl. car, right?

You're tach problem is that it is set for the engine that was in there, 4 and 6 cyl tachs will read retardly high with a V8.

If you have an '87-'93 car, it needs to be swapped out for a 5.0 unit. If you have an '79-'83 car, pull the instrument trim and there will be a little switch marked 4-6-8 on the bottom right of the tach... Use a flat screwdriver to turn it to "8". Problem solved.

hllon4whls · Apr 27, 2006

So whats the story? Any update wazazzle?

We know you cant be driving that thing, let us know if you found anything yet.

67coupe · Apr 28, 2006

What size wheels and tires?

FORCED2DV8 · Apr 28, 2006

Did you ever swap tranny's, Some of the earlier t5's from the 4 cyl had extremly low ratio's. If you have one of these plus the 3.73in your rear end. Your rpm 's will be quite high, A friend had the same problem but he had 3.27 gears with the 4 cyl. trans but the end ratio was more like 3.55 3.60. Hope you figure it out.

67coupe · Apr 28, 2006

Also, you stated that at 60 mph, you measured 1 mile in 1 minute using the odometer. That is not the correct way to measure the speed you are truely traveling at. The speedometer and odometer are driven off the same gear/cable. Using this method will not tell you if you have the wrong gear on the cable or in the transmission. To measure your true speed, do the following.

Using the mile markers on the side of the interstate, measure the time (in secs) it takes you to travel one mile. Then use this formula:

Speed = 1/(time in secs) x 3600

Or just get a friend to drive next to you and compare speedometer readings.

Good luck!

VG30DE · Apr 28, 2006

My guess is he's in 3rd gear. He said its a 5 spd....

maverick0716 · Apr 28, 2006

VG30DE said:
My guess is he's in 3rd gear. He said its a 5 spd....

Yeah, that would pretty much make sense........I think mine's around there with 3.73's in 3rd gear.

Dean85GT · Apr 28, 2006

I bet its a 4 banger T5

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