Hi,
I have 1991 firebird LO3 in which the previous owner made some changes to the rear gear (original transmission, but 16" tires instead of 15") and calibrated a 140mph speedometer to it. I changed back to the original 120mph speedometer (91-92;electrical)now it is not marking the proper speed. My questions is:
1- How can i figure out/verify the rear gear in the car?

Thanks!

 

Take the rear cover off and there will be some numbers on the ring gear like 41-10 which is 410.

 

Thx. Will do that today.

 

Most after market gears are not marked like that. You will have to count the teeth on the pinion gear and ring gear. Divide the number on the ring gear by the number on the pinion gear for the ratio. 41/11=3.73

 

essentially my own calculation...

 

Block front wheels ,jack rear of car ,P brake off and out of gear
Mark the driveshaft and tire , rotate the driveshaft and count how many turns of DS it takes to get 1 rev of tire ;
or turn the tire once and count how many times DS turns
Gives ballpark rear ratio
Exp
3 3/4 turns = 3.73

 

Both tires must turn together exactly 1 turn (good working posi), or leave one tire on the ground and turn the other one exactly 2 turns (standard differential).

 

standard differential= stock
correct?

 

Yes, spins only one wheel.

 

got it... i like this method better.

 

Ok. I did the test. When i spin the wheel, the other wheel spin the opposite direction.
For 1 wheel cycle, the driveshaft spin about 1 1/2 time. Is this then a standard rear end?

 

You are going to have to hold the other wheel still while you do this. Turn one wheel exactly 2 turns while the other wheel is still. That will give you your gear ratio.

 

Perfect will do tomorrow, and let you all know

 

Yeah, but you're still grasping at straws here. The opposing spinning tires could be a very worn out posi. Or a gear bias (like Torsen) diff.

If you put a known-working 120mph unit it and it's off, he didn't really "calibrate" anything as much as just "make it work. Maybe accurately. Maybe not."

The speedo gears are chosen originally so that based upon the rear gear and tire size, the VSS is spun about 1001rpm @ 60mph.

The car could have an incorrect VSS, incorrect ratio speedo gears, the original VSS buffer wiring could be altered.

Something is "off" since you don't know the reason for a stock 120mph speedo to not work correctly. Yet.

Start doing the rear fluid and gasket so you can count the gears. Then get the speedo gear counts. Post those up and the tire size. Then you'll better know why the speedo is off.

 

Have you attempted to find the RPO Code sheet for the vehicle.. They were usually put under the glovebox lid or by the rear spare tire assembly. This will tell you exactly what was put in the car at the date of manufacture, providing someone hasn't changed the rear end.

 

Thanks ibmtech--Yes, the original rear gear is 2.73.

jmd--is this applicable to mechanical and electrical speedos?

 

Which point? The speedo gears? Yes. Third gens all had mechanical speedo gears. Instead of a sleeve for a speedo cable connection, elec. speedo third gens got a VSS which is physically driven by a speedo gear. So Elec. speedo Berlinetta, all 86-up birds, all 90-92 camaros. A VSS buffer was used to divide the signal except on 90-92 SD TPI f-bodies, which did it within the ECM.

 

i agree will need to count the gears. Yesterday, i got underneath the car, and i found the the ID number of the transmission, according to the service manual suggested locations, but could not find any number written on it. I wonder if he messed with transmission as well?

 

I did gear head suggested. Conclusion: 2.73, almost 3 turns.

 

Most stock 3rd gen Firebirds whether v6 or v8 got crappy gears for acceleration like 2.73s, my '87 Iroc had them, lousy take off but good top end, it only takes an extra minute or two to get there Haha

 

agree