Help to determine speedo cable gear - Drivetrain - Firebird Nation

Jump to content


FirebirdNation.com is the premier Pontiac Firebird forum on the internet. Supporting Members do not see the above ads.
Photo
- - - - -

Help to determine speedo cable gear

Speedometer Speedometer cable

  • Please log in to reply
6 replies to this topic

#1 Brada

Brada

    Firebird Tire Changer

  • Full Member
  • PipPip
  • 61 posts
  • Gender:Male
  • First Name:Brad
  • City:Moorhead
  • State or Province:MN
  • Country:USA
  • Interests:Rugby, caribbean islands, travel, F1 racing, maps.
  • Year: 1975
  • Make: Pontiac
  • Model: Trans Am
  • Engine: 455
  • Transmission: '73 M22 4 sp

Posted 27 November 2020 - 06:58 AM

In short, my 75 has a 73 muncie 4 sp, tail housing cast is 3978764 - 32 spline output. Just put new BF Goodrich TAs on it, rears are P245/60R15. Diameter 26.6 inches, circumference 83.4 inches. The car came with 3:23 rear differential, it had 40 years of life before I assumed caretaker responsibilities, so I am not sure if it has been changed. It seems to have a lot more rpm on the highway than my old 75 400 4 sp (also a 3:23 rear)- but that was a long long time ago in a different life. I saw a post reply from js292 in 2018, a 3:23 should have 3 1/4 driveshaft revolutions per 1 rotation of rear tire. Need to check that, when car comes out of storage and the geese head back north.

Problem; on the highway my speedometer reads 60 and in reality I am doing 70 - 71. When I pass those info signs the city puts out that flash your speed, speedometer is around 25 and sign is flashing 30.

Any help for determining what speedometer cable driven gear I need to purchase would be very much appreciated.

Edited by Brada, 27 November 2020 - 07:11 AM.


#2 b_hill_86

b_hill_86

    Firebird Oil Changer

  • Full Member
  • PipPipPip
  • 160 posts
  • Gender:Male
  • First Name:Brian
  • City:Toledo
  • State or Province:OH
  • Country:USA
  • Year: 1977
  • Make: Pontiac
  • Model: Trans Am
  • Engine: Pontiac 400
  • Transmission: 4 speed

Posted 27 November 2020 - 05:39 PM

Honestly, a good place to start would be pulling the current driven gear to see what you have. Then you can use that to help determine your drive gear which will be necessary to determine the proper driven gear.

Another option could be to use an adapter to make the correction but those seem to be around $90+. Driven gears are much cheaper.

#3 JoePeek

JoePeek

    Firebird Master Tech.

  • Full Member
  • PipPipPipPipPipPipPip
  • 1503 posts
  • Gender:Male
  • First Name:Joe
  • City:Westfield
  • State or Province:IN
  • Country:USA
  • Year: 1971
  • Make: Pontiac
  • Model: Trans Am
  • Engine: 455 HO
  • Transmission: 4 speed

Posted 28 November 2020 - 02:01 AM

There are 5 variables that are all related here: rear tire height (=26.6), what gear you are in (4th=1:1),the rear axle ratio, the engine RPM, and car speed. 

 

If you know 4 out of 5, you can solve for the one you're missing.

 

Right now you are not sure you have a 3.23 axle rear, and you are not saying what the RPM is when you are going at whatever speed.

 

Since a Muncie 4 speed has a 1:1 4th gear, that is known, and your tire diameter is known....those two are for sure.

 

If you get on the highway in 4th gear, and can write down that you are going some true MPH, at some RPM...then you can calculate the rear end gear, and go from there to figure out what color speedometer cable gears you need.

 

Right now all you know is your speedometer cable is turning too slow. As said above, if you check to see what speedometer gears you have know, you may be able to get a few gear colors that are the next few up and try them until you get real close, or do the RPM vs. speed test and calculate it straight out.


Edited by JoePeek, 28 November 2020 - 02:02 AM.


#4 ZumpTA

ZumpTA

    Firebird Master Tech.

  • Administrator
  • 2330 posts
  • Gender:Male
  • First Name:Tony
  • City:Holiday
  • State or Province:FL
  • Country:USA
  • Year: 1993
  • Make: Pontiac
  • Model: Trans Am
  • Engine: LT1
  • Transmission: 6-Spd

Posted 28 November 2020 - 05:24 PM

There are 5 variables that are all related here: rear tire height (=26.6), what gear you are in (4th=1:1),the rear axle ratio, the engine RPM, and car speed. 

 

If you know 4 out of 5, you can solve for the one you're missing.

 

Right now you are not sure you have a 3.23 axle rear, and you are not saying what the RPM is when you are going at whatever speed.

 

Since a Muncie 4 speed has a 1:1 4th gear, that is known, and your tire diameter is known....those two are for sure.

 

If you get on the highway in 4th gear, and can write down that you are going some true MPH, at some RPM...then you can calculate the rear end gear, and go from there to figure out what color speedometer cable gears you need.

 

Right now all you know is your speedometer cable is turning too slow. As said above, if you check to see what speedometer gears you have know, you may be able to get a few gear colors that are the next few up and try them until you get real close, or do the RPM vs. speed test and calculate it straight out.

 

A good way to get your true speed when the speedometer is wrong is GPS.  On Android, I use DigiHUD.  It's free, no ads, etc.  Only basic functionality, but "good enough".  I know I'm probably saying the obvious, but it's so obvious many forget, so, just throwing it out there as a reminder.   ...those "your speed is" radar decices on the road aren't always calibrated, so don't trust them.  Since they aren't admissible as evidence, they don't get the care they should.  They're great to give you an idea of your speed, but certainly not as authoritative as a calibrated radar gun or GPS.


"There are two major products that come out of Berkeley: LSD and UNIX.
We don't believe this to be a coincidence."
-- Jeremy S. Anderson

"Man is the best computer we can put aboard a spacecraft...
and the only one that can be mass produced with unskilled labor."
-- Wernher von Braun


#5 b_hill_86

b_hill_86

    Firebird Oil Changer

  • Full Member
  • PipPipPip
  • 160 posts
  • Gender:Male
  • First Name:Brian
  • City:Toledo
  • State or Province:OH
  • Country:USA
  • Year: 1977
  • Make: Pontiac
  • Model: Trans Am
  • Engine: Pontiac 400
  • Transmission: 4 speed

Posted 28 November 2020 - 06:30 PM

Dont forget though that the stock tach isnt necessarily 100% accurate. Not a big deal of checking the difference between a 2.41 and 3.08, for example, but the difference between 3.08 and 3.23 could be lost in an inaccurate tach reading. Just something to keep in mind.

#6 Brada

Brada

    Firebird Tire Changer

  • Full Member
  • PipPip
  • 61 posts
  • Gender:Male
  • First Name:Brad
  • City:Moorhead
  • State or Province:MN
  • Country:USA
  • Interests:Rugby, caribbean islands, travel, F1 racing, maps.
  • Year: 1975
  • Make: Pontiac
  • Model: Trans Am
  • Engine: 455
  • Transmission: '73 M22 4 sp

Posted 29 November 2020 - 12:59 PM

The TCI website calculator didn't work becuase I don't know either the drive or driven gear teeth (so user error). From the good advice above, need to get accurate speed in 4th & rpm to determine rear end gear. Leaning towards it is still factory 3.23, but you never know for sure.

I see the driven gears are pretty cheap, $10 to $20, and 17 teeth was lowest count in the classic indust catalog. So when I pull the driven gear I order one (or two) that have fewer teeth than mine does, correct? I said in previous post before starting this one, my neighbor across the street is a state trooper, he could check me with his radar gun. A really nice guy.

Thanks guys!

#7 b_hill_86

b_hill_86

    Firebird Oil Changer

  • Full Member
  • PipPipPip
  • 160 posts
  • Gender:Male
  • First Name:Brian
  • City:Toledo
  • State or Province:OH
  • Country:USA
  • Year: 1977
  • Make: Pontiac
  • Model: Trans Am
  • Engine: Pontiac 400
  • Transmission: 4 speed

Posted 29 November 2020 - 01:38 PM

Yes fewer teeth would increase your speedometer reading. I bought a set of driven gears off eBay for my T10. https://www.ebay.com/itm/153437061125
Comes with several gears plus the housing and o-rings.





Also tagged with one or more of these keywords: Speedometer, Speedometer cable

0 user(s) are reading this topic

0 members, 0 guests, 0 anonymous users