Topic: New speedo crazy

[Crisis]

Hello everyone,

I am speaking to the group to see if anyone can help me ...
I'm French. I am restoring a 79 Trans Am which has always been European. She had a speedo in Km/h with a bouncing needle.
After having replaced the speedo (by a 100mph one), the two cables (from the tranny to the cruise and from the cruise to the speedo) and the cruise control, the needle no longer bounces but does not indicate the right speed:
The needle moves with a little lag after kick off. Below 70km/h, it is too low by 10km/h and beyond 70km/h it is too high increasing, finally to indicate 160km/h when I am only 110 to the gps.
Would anyone have an idea of the problem?

Thank you

My car : 403ci olds with th350 - tires 4x 275 40r17

[Stringer]

I am not sure, but it could be that changing from a km/h speedometer to mph speedometer has resulted in the speedometer drive gear, at the transmission, no longer being the correct operational ratio.  I would change out the drive gear (for one with less or more teeth) then check it. Should cheap and easy to do.

[Crisis]

Hmmm... thanks. I've checked that.
And after some further investigations, i've found this url (a speedo gear calculator) :
http://www.tciauto.com/tc/speedometer-gear-calculator
Maybe helpful... I'll try

[Crisis]

Hi,

I've a problem. I can't identify my rear end axle.
There is nothing writen on it except "1254333" and "D0199". I've looked everywhere but there is nothing more casted on.
So.
I know that my 1979 T/A 403ci olds TH350 rear end axle is stock. It has a chrome finish 10 bolts.

Anybody knows what is the ratio of my rear end axle, please ?

[Wallington]

There is no way to tell simply by looking at it, and no way to say if it is even stock, likely, but then half the swaps owners are not aware of. The ratio is not cast on, it is stamped into the axle tube. The chrome cover is not original. Do the old spin the wheel and count the driveshaft turns, but that's based on also knowing diff is not worn and if it is posi or standard. To be sure, just pull the cover, check the old oil for any rubbish, count the teeth or check the stamped numbers on the sides, add new gasket and fresh oil and know for sure.

[Crisis]

Well, well, well... i'll do it.
Thank you Aus78 !

[Stringer]

The stamps on the axle tube list the ratio -- but in code.  Here is a link that list identifying codes for Firebirds/Trans Ams. The differential section is toward the bottom.

http://www.78ta.com/Pages/etrcodes.htm

[Crisis]

Thank you. Sadly, there is nothing stamped on the axle tube.
I'm going to open the rear end to be sure...

I've noticed 4 numbers casted on the speedo gear housing on the tranny : 40, 41, 42 and 43.
Means that this housing can only support 40 to 43 teeth driven gears?

I need to check into my rear end but it seems to be a 2.41.
According this calculator : http://www.tciauto.com/tc/speedometer-gear-calculator , i should use a 36 teeth driven gear.
If it is, must I replace the speedo gear housing too ?

[Wallington]

That's right, ratios like 2.41 aren't obtainable with the 40-45 gear housing. Luckily, the lower number housing is far more common, hold onto that 40-45 though. For 2.41 they often used 35/19 speedo gears or similar, which is already right at the end of what was even available.

Seems the transmission has been changed at some point causing the speedo error, so you'll not only need the housing but likely both gears to suit. There's also the possibility of the ratio adaptor box with speedo cable either end for fine tuning but easier just to use the correct parts to start with, once you know what they are. If you unplug the speedo cable at the gear housing, mop up a few trans fluid drips, you should be able to see the colours of the 2 gears inside to at least confirm what the transmission is set up as originally.

And remember, the BOP Th350 used behind the 403 uses TH700 speedo gears and parts, ignore all TH350 references, unless BOP specific). I've used this guy a few times over the years, very helpful and has a good listing of parts.

https://transmissioncenter.net/product-category/gm/700r4-transmission/

[Crisis]

It's good, I opened the rear end, drained, cleaned, closed and filled with 80W90.
Verdict: It's a 3:23 POSI!
Into the tranny side, I have a yellow drive gear (19 teeth) and a dark grey driven gear (44 teeth).
The problem is that for a 19 teeth, with wheels in 25.66 'and a diff in 3:23, there is no adapted driven gear. It's out of range.
So i have to replace the drive gear in the tranny or to change the tranny! ^^
And since I intend to put a TH700r4 ... 

[Wallington]

Yes, somewhere along the way it's had trans swap, complete with original cogs possibly from a 3.08, or they were also always wrong and mixed up.

A BOP th700 ?

[Crisis]

BOP or not, i don't know cause i'm a newbie into this world! X)
The th700 which will match with my 403 i presume.
Essentialy to reduce gasoline consommation and improve driving pleasure on highways.

(hope i've understood your message, my english ain't very good)

[Wallington]

I hit send too early. Meant to say there are no BOP Th700 so would need an adaptor platr for different bolt pattern. The 700 may use the same speedo gears as the BOP for same setup as it only adds an overdrive extra gear, but you'd need to check for sure.

[Crisis]

Replacing the trans is the only thing that i'll can't do myself.
I'll check everything needed with my mechanic which is specialised in american cars.

But i'm surprised cause i've seen on many sites and forums that TH700r4 has same bolt pattern as TH350.
Strange... probably haven't understood what i've read. It occurs often !!! ^^

[Wallington]

Only a chev th350...the 403 uses the other version BOP.