I changed rear end axle with the same gear but new rear axle has posi. i pulled out 3.23 non posi and installed 3.23 posi) Rebuilt carb and addded Petronix. Now when i go down the road speedo is off about 15 mph but the tach reads the same. This doesn't make sense???? Not sure why? The only thing is i tighened up the speedo cable as it was a little loose and was leaking fluid?

 

How do you know it's out by 15 mph? Check it against a GPS? There shouldn't be any change at all in the speedo readings going from open to posi with the same ring/pinion set.

The speedo cable shouldn't be leaking anything at all. Are you sure it's not the sleeve seal that was leaking?

 

Did you re-run the same gears or did you swap the crown & pinion?

If you changed gears then either the original set was not a 3.23 or the set you put in is not a 3.23.

If you re-ran the gear set then I am lost.

 

yes was check against GPS on droid phone, not sure about leak but trans fluid was on it and was a little loose, just tightened? That what was strange as i thought gears where off??? Until RPM's were checked. RPM's are 3300-3400 before on 235/60/14 Bfgoodrich t/a's and are 3300-400 now on same tires? What color speedo gear should i have for 3.23 gears and i will check tomorrow?

 

i swap entire rears!

 

Are you 100% certain that both crown & pinions were for 3.23s? If you were comparing axle codes that might be the issue.

You will need to allow for tire height before choosing the speedo gear.

 

Yes, did you actually either a.) count the teeth on the gears, or b). count the wheel revolutions per driveshaft revolution on both rears?

If not, I would say that while you thought you replaced a 3.23 with a 3.23, cold reality indicates otherwise.

- Eric

 

Is your car a 350 or 455? Need a point of clarification. Did you change out the entire rear axle in the car, or just the carrier? Your post says 'entire rear' which is what I'm curious about. Since you are running the same size wheel/tire combo, it makes no sense that your speedo is now out of whack.

Dave - the same wheel/tire combo used as before with reported 3:23 open and matching RPM's

 

I get what you're saying Eric and it makes sense. For a 15 mph change in the speedo with the same RPM, that would be at least 2 gear sizes wouldn't it? I was under the initial impression that the carrier was the only change and the same ring/pinion used on the new posi carrier. If it was a complete axle change, the axle stamping could easily be different from the actual gear. My 72 axle code is SA which is 2.73 and my actual gear set is 3:42. I think that was similar to your point in your comment?

 

Similar indeed. Forsooth perhaps identical.

When you "know" that something is the case, but all of the evidence indicates the contrary, it is sometimes wise to revise your initial assessment (unless you work in politics).

- Eric

 

Forsooth, foresaken, forelorn and forewarned. Gotta love Fore!!! Ok, nuffa this...

 

I think you missed "foresk..." Oh, never mind.

- Eric

 

Maybe I missed it but is this 15mph speedo change fast or slow from before?

 

Swinger?

 

Yeah, Baby! ♂



- Eric

 

ok i drove again today and the rpm's are more than what was thought but not by more than 200-300 rpm's. Speedo is off seems like the original diff was changed somewhere before.(very disturbing!) On the factory tach( i know may not be acurate) was at 3600 rpm's with 235/60/14 tire which is small-at 55 mph. What is the ratio from pinion teeth to ring gear teeth? I will pull cover off of old rear. And you divide which way?

 

RE; Getting the correct gear.

This seller looks to have sold many and says they will help you select the one you need.

http://www.ebay.com/itm/42-tooth-700...c24ebd&vxp=mtr

 

the pinion into the ring gear, pinion 10 ring gear 30 =3.0 to one. good luck

 

I'm sorry, but something's not right here.

If you have 235/60x14 tires, they are 78.9" in circumference, turn 803 times in one mile, and 44,165 times in 55 miles (which takes 1 hour at 55mph).

If your engine is turning at 3,600 RPM for one hour, it will turn 216,000 times.

216,000 ÷ 44,165 - 4.89, so therefore, if your tachometer is accurate, you have a 4.89:1 rear end.

Once again, let's look at reality first, and then figure out the truth.

Any chance your tach is off?

- Eric

 

My money is that he knocked off the modulator hose when tightening the speedo cable, and is making it WOT shift into second and it's staying there. Eric, your numbers are right. If he were in second gear, with a ratio of 1.48 to 1, his rear for that number of revs at that speed would be ~3.30, which is close enough for the girls I date to 3.23 to make me suspect he's in Super Range.

 

Ha ha. Nice catch.

- Eric

 

I will check the modulator valve to make sure it is connected and also put a vacuum test on the rubber hose. The tach may not be accurate. I purchased the rear from 507 Olds and I trust he has the right gears that was advertised when I purchased it.

 

Ok I was reading up on the speedo gear change, do I have to change the housing going from a 35 tooth gear to a 45 tooth gear? Also are the housing the same between Chevy turbo 400 compared to BOP turbo 400's?

 

You could just count the upshifts as you accelerate.

- Eric

 

i did check and it upshifts to the 3rd gear.

 

One of them was not a 3.23, it's the only thing that makes sense.

Is it off by 15 MPH? It would be off by a percentage, not a set MPH.

Example, when I went FROM a 2.73 TO a 3.42, the difference was 27%, and my speedo read 27% too fast