changing my spedo gear to get right after rear end swap - Hot Rod Forum : Hotrodders Bulletin Board -- Hot Rod Forum

Register FAQ Search Today's Posts Unanswered Posts Auto Escrow Insurance Auto Loans
Hot Rod Forum : Hotrodders Bulletin Board > Tech Help> Transmission - Rearend
User Name
lost password?   |   register now

LinkBack Thread Tools
  #1 (permalink)  
Old 09-22-2005, 12:41 AM
montea's Avatar
tryn to learn
Last photo:
Join Date: Dec 2003
Location: Alberta
Posts: 844
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
changing my spedo gear to get right after rear end swap

in my 82 grand prix

2.29 (26.10 overall diameter inchs)

TH350 (original but rebuilt)
295/50R15 (26.61 overall diameter inchs)

i found a shop that deals just with spedometers in the city but they cannot tell me wich gear i need.

how do i figure out wich gear i need to buy? do i need to find the teeth on the current gears and go from there or is it possible to just find out with my new setup?

i read the sticky thread from m&mcustom, i didnt find anything on there i needed tho, or maybe i missed?

Reply With Quote Quick reply to this message
Sponsored Links
  #2 (permalink)  
Old 09-22-2005, 08:34 AM
Last wiki edit: Vacuum brake bleeder set up
Join Date: Nov 2003
Location: So Cal
Posts: 1,245
Wiki Edits: 8

Thanks: 0
Thanked 0 Times in 0 Posts
This is a quote from Steve Chin on CK5
The correct way for a speedo shop to calibrate a speedo is to ensure that the odometer reads correct at 1000 revolutions per mile, then make the speedometer head agree with that by either moving an adjuster in the speedometer head or by changing the magnetism of the flying magnets in the head.

That said, you first need to figure out what gears you have in your transfer case (you already know your tire OD and final drive ratio). To calculate what gears you need, you first need to know the Speedometer Gear Ratio (SGR).

Calculate that by:

FDR = Final Drive Ratio
TOD = Tire Outside Diameter

SGR = 63360*FDR/(TOD*3141.597)

In this case, with 38" tires and a 4.10:1 final drive, the SGR = 2.176030384

Next, you need to know the tooth counts on the drive gear and driven gear. The only way to find out is to remove them and count.

If you divide the number of teeth on the driven (output) gear by the number of teeth on the drive gear (the gear on the output shaft) and that result does not agree somewhat closely with the SGR, you need a ratio adapter (or you need to change one or both of the gears).

In order to determine the ratio of the drive adapter you need, you simply:

SOG = Speedometer Output (driven) Gear
SIG = Speedometer Input Gear
SGR = Speedometer Gear Ratio

Adapter Ratio = SOG/SIG/SGR

Note that some adapter ratios might be overdrive (greater than one) and some might be underdrive (less than one).

Clear as mud?

Speedometers and odometers are calibrated for 10,000 revolutions in a mile. You need to calibrate the gears in the transfer case to obtain a gear ratio of 2.35:1 to get the correct odometer reading, then calibrate the speedometer head to jive with that. Most likely, you will not be able to achieve the exact gear ratio and you may need to go with something that's somewhat out of whack and use a drive ratio adapter (I have a 20.2% overdrive adapter on mine that was installed by the factory because the factory set my TC up for 3.73s and 30" tires).

FWIW, this is the formula for speedometer gear ratio: (63360 * Axle Ratio) / (3141.6 * Tire OD in inches) = SGR.

Take the SGR and multiply it by the number of teeth on the drive gear to obtain how many teeth you need to have on the driven (output) gear: SGR * DGT = OGT
Reply With Quote Quick reply to this message
  #3 (permalink)  
Old 09-25-2005, 02:08 AM
M&M CUSTOM's Avatar Moderator
Last wiki edit: Transmission identification
Last journal entry: 1988 Suburban, the project continues.
Last photo:
Join Date: Mar 2003
Location: Northwest Pennsylvania.
Age: 44
Posts: 2,236
Wiki Edits: 9

Thanks: 0
Thanked 2 Times in 1 Post
The old gears you had were at or close to a 1.79 reduction ratio.

The new gears will need a gear reduction of 2.05 to 1.

This will be MUCH easier if you knew what gears you already have in the transmission.

If there is any way you can see either one to tell me the color (they are color coded) you could save the hassle of pulling the tailshaft or speedometer housing off just to take a tooth count.

The 10 tooth (red OR purple drive gear on the transmission output shaft) and 18 tooth brown driven gear give a ratio of 1.8 (close enough for the necessary 1.79 needed).

Either way you will need to change both gears (just in case the drive gear is already worn and the cost isn't that much) to a 10-tooth red or purple drive gear, and either a 20-tooth blue driven gear, or a 21-tooth red driven gear.

Both gears are equally split in the difference from technically "exactly" what you need, and the 20 tooth gear will read "slightly" higher around 1.5 mph faster at 55 mph, and the 21 tooth gear will read the same only 1.5 mph less at 55 mph.

It's just easier knowing what you already have, because the TH-350 has two different gear sets, you can read more on this at TCI's speedometer information page. You stated an 82 Grand Prix, so this does make a difference- the Chevy and the BOP transmissions have different speedometer housings.
Reply With Quote Quick reply to this message
  #4 (permalink)  
Old 10-04-2005, 03:06 PM
montea's Avatar
tryn to learn
Last photo:
Join Date: Dec 2003
Location: Alberta
Posts: 844
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
ok finnaly got time to check out the gears in it... the small gear is 19tooth and light brown i believe and the inside gear on the tailshaft i thought i would remeber but soon as i put the spedo cable back in i forgot! but it looked like it was grey or light pink or somthing, hard to tell when its all covered in tranny fluid. but i do think its a 7 tooth

i got the car saftied today and is on the road tommarow, untill i can swap out those gears aproxx how much is my spedo going to be off the way it is right now? it will show i am going slower then i really am correct? i just dont want to get a speeding ticket, i can follow all other cars and go same speed but no one goes the speed limit.
Reply With Quote Quick reply to this message
  #5 (permalink)  
Old 10-04-2005, 03:33 PM
montea's Avatar
tryn to learn
Last photo:
Join Date: Dec 2003
Location: Alberta
Posts: 844
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
ok i read your post over a couple times and checked that website out wich has a awesome calculator thing and i think it would be best for be to go with a 10 tooth drive gear and a 20 tooth driven, id rather it say faster then slower.

my tranny is the original in my pontiac but i belive its the exact same as a chevy one because its a chevy bellhousing pattern and only 1 pattern plus it was originaly hooked to a chevy 267

if you could just confirm the 10T drive and 20T driven before i go pick them up that would be awesome! thanks M&M!
Reply With Quote Quick reply to this message

Recent Transmission - Rearend posts with photos
Gear in this thread - Powered by O'Reilly Auto Parts

Quick Reply

Register Now

In order to be able to post messages on the Hot Rod Forum : Hotrodders Bulletin Board forums, you must first register.
Please enter your desired user name (usually not your first and last name), your email address and other required details in the form below.
User Name:
Please enter a password for your user account. Note that passwords are case-sensitive.
Confirm Password:
Email Address
Please enter a valid email address for yourself.
Email Address:
Please select your insurance company (Optional)


Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
Thread Tools

Posting Rules
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

Similar Threads
Thread Thread Starter Forum Replies Last Post
this is a saver.... GM build codes TooMany2count General Rodding Tech 24 11-23-2009 09:36 AM
air ride, ifs, engine swap, rear end blake Suspension - Brakes - Steering 1 11-19-2004 05:31 AM
rear end swap for 1950 chevy w/ 350 blake Transmission - Rearend 2 10-10-2004 08:51 AM
Rear end swap for '53 Chevy truck Bruce H Transmission - Rearend 4 12-01-2002 11:43 AM

All times are GMT -6. The time now is 10:27 PM.

Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2016, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0 PL2
Copyright 1999 - 2012. All Rights Reserved.