98t595

i can check but dont feel like jacking up car just to check ... when someone probably knows here. did a gear change and need to correct the difference in the speedometer..was directed to a great site but i need to know the drive gear to find the right application.. my car is a 92 z28 305- 700r4 auto was 273 now 373 gears... so is anyone pround to know the factory drive gear tooth count? thanks

camaronewbie

I believe we have the grey (15tooth) drive gear from factory - it's been awhile since I swapped mine and I'm not 100% sure. There are several stickies at the top of this forum with gear info.

jmd

iTrader: (4)

700-R4 uses a 15 or 17 drive gear.

With stock 3.73s, the 1984 Z28 used 15/45 speedo gears in the 700-R4.

Atilla the Fun

Stock speedo gears vary, but a 15:45 is only right for 3.73:1 if you're running 255/40R17s. If you're running 245/50R16, 225/60R15, 245/45R17, 275/40R17 315/35R17, 275/35R18 or 245/40R18 tires on the rear, then you need 15:44 gears. Current GM part numbers are in my sticky thread. If you're running 235/55R16 or 235/60R15 or 335/35R17 or 255/50R16 or 235/50R17 rear tires, then you need 15:43 gears.
2.73 geared cars did not come with 15 tooth drive gears, and you cannot calibrate for a 3.73:1 with any drive gear other than 15 tooth, but it's cheap and easy to change.

jmd

iTrader: (4)

15/43 isn't what GM used with a 215/65R15, nor is it accurate with a 235/60R15 tire. I've had that tire size with multiple T5s, multiple 200-4Rs and multiple 700-R4s. I've checked the odometer against mile markers, the speedometer against police radar, against other cars, swapped speedometers and I didn't even spend the $$$$ that GM does in vehicle R&D.

Tires compress on load and more teeth on the driven gear (than what all the online speedo gear calculators recommend) or similar compensation in the ratio of gears is required to be accurate.

I guess I'll have to post this again (and again)
Online speedo gear calculators are inaccurate.
The speedo gear combinations used by the OEM are accurate.

1984 Z28:
20168.0672268 / 26.0 * 15 * 3.73 / 1000 = 43.4.... (GM used 45)

1986 Turbo Buick:
20168.0672268 / 26.0 * 11 * 3.42 / 1000 = 29.18... (GM used 30 driven)

There are lots of examples that can be plugged into the formula you posted which give different results than what was used in GM cars.

As for the OP, a shorter stock (245/50R16) tire than what was used in the earlier 3.73 third gens means the tires will turn more revolutions per mile. This means more teeth are required on the driven gear. There is no 46 tooth driven gear for the 700-R4 so the 45 is what you use.

Atilla the Fun

How can you be so ignorant? a 215/65R15 tire, which new and unloaded measires 26.0" diameter, will travel 81.68 inches per rotation, no matter how much wiight you put on it. The formula I use comes directly from GM engineering. 20168.0672268 divided by new unloaded tire diameter, times number of teeth on drive gear, times axle ratio, divided by 1000. The end. Cannot be argued. Period. Irrefutable.
Why? Because there always will be 63360 inches per mile. Because speedometers are calibrated for 1000 rpm at 60 mph. For the example above, 63360 divided by 81.68 is 775.7101. If you do 20168.06723 divided by 26, you get 775.695. Close enough.
Now, stop re-posting your misinformed opinion, the facts stand forever. You cannot if you refute them.

AaronIROCZ

iTrader: (5)

Lets keep this civil guys!!

jmd

iTrader: (4)

GM used different speedo gear combinations than what you say are correct.

https://www.thirdgen.org/forums/tran...ive-gears.html
The first attachment from the Firebird shop manual shows T5 and 700-R4 3.73 combinations with 215/65R15 tires. Every one used 7 / 21 or 15 / 45 speedo gears.

This is all I've seen out of bone stock third gens with that combo. Every 215/65R15 / 200-4R / 3.73 GM car I've touched had 10 / 30 speedo gears stock.

PS, it's 1001 @ 60.

Your formula does not work accurately.

Guest

Look, the factories, ALL of them, always calibrate production speedometers to be SLIGHTLY optimistic, to be sure you don't get speeding tickets if you're not really speeding.
That's why they don't use the combos suggested by the formula, and that's why the common formula specifies 1001, not 1000. Ford even goes so far as to give the formula as 22000 rather than 20168, IIRC. I remember seeing it in my old paper copy of the FRPP catalog.
Atilla, if what I just typed isn't exactly correct, then please straighten it out.

Redchevy808

Man nobody answered the first question, he has 2:73 gears and you guys said grey 15 tooth came on all but 2:73 geared cars. What's he got?

The Project

iTrader: (6)

Post is a little old.

Thought some auto cars also had 3:23 depending on the performance options.

Edwardgp

Digging up bones.

vetteoz

iTrader: (13)

After 4 years I hope he has worked it out

jmd

iTrader: (4)

You have a 50% chance at this point.

graysmoke

Digging this up because I have the same question as the OP. It never got answered and the last post said 50% chance. I can crawl under the car and check but maybe someone can save me some time and tell me what tooth drive gear came in my factory 91 700r4 with factory 2.73?

Added: I found my own answer. thanks

graysmoke

For anyone who wants to know, the answer can be found in your RPO code.
Mine is:

D3W - GEAR-SPEEDO DRIVEN 25513044

Cross your RPO code to the factory driven gear part number. Once you have that use the attached chart to find what drive gear came with your combination. take note of the driven gear and speed sensor part number

for other years use the link
source:
http://www.gmpartswiki.com/w/25513044&book=228

88bullet

I have 410 gears with 20 inch wheels says I need a 47 tooth driven gear I put a 45 tooth driven Gear in but need two extra teeth my speedometer is off by 5 miles an hour over what it's actually doing I heard more teeth slows down a speedometer but I don't know if I have a 15 tooth drive gear or 17 so I don't know if I should change the drive gear to a 19 or 17 to get that correct does anybody know what was stock on a 88 Camaro with a 305 as far as the drive gear goes thanks in advance