Speedo Gear so confused
Thread Starter
Racer
Joined: Jul 2007
Posts: 269
Likes: 0
From: WV
Speedo Gear so confused
I have been doing some searching online and keep getting more and more confused on what I need... Right now my 75 has a turbo 400 with 3.08 gears and the stock speedo gear. I have a new 3.70 rear I want to install but I am confused on what speedo gear I need for it... Seems like every search I do gives a different answer on what to get.
Safety Car
Joined: Oct 2004
Posts: 3,926
Likes: 739
From: Youngstown Ohio
It depends on which "stock" speedo gear you have. You may have a 15t or 17t speedo drive gear, which gives a couple choices for the driven gear. You have to look at the color or count the teeth to be sure.
Here is the calculator to determine which one you need:
http://www.bgsoflex.com/speedo1.html
Here is the link for info (scroll down to TH400)
http://www.transmissioncenter.net/sp...Fcnc4AodDjLF9Q
Here is the calculator to determine which one you need:
http://www.bgsoflex.com/speedo1.html
Here is the link for info (scroll down to TH400)
http://www.transmissioncenter.net/sp...Fcnc4AodDjLF9Q
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
At a minimum, the speedo output gear in the tailshaft of the transmission needs to be changed.
Note: If you are lucky, you will only have to change that one gear; but, because of the spread of ratios you are changing, you may have to change the speedo drive gear [on the output shaft of the tranny] AND the speedo output gear to get the job done properly.
The ratio change you are making is 3.08/3.70 = .83 . Therefore, you need a speedo output gear that has only 83% of the number of teeth on your present output gear. Remove the speedo output gear housing from the tailshaft of the transmission, pull the gear off and count the teeth on it (it should also have the number of teeth cast or stamped into it somewhere). Multiply that number of teeth by .83; the result is the number of teeth on you gear that you need.
There are 3 different output shaft drive gears that are available. Each of those drive gears can accept several output gears and still be properly matched. But, each can be matched with different speedo output gears. So, depending on the speedo output gear you need, that selected gear may require that the internal gear be changed, also. If that is the case, then you will need to use a "Speedo Gear Calculator" program to determine the proper combination of speedo drive gear and speedo output gear that you need for your car.
Hopefully a speedo output gear with 17% fewer teeth will be the only change that is needed to get you going again.
Note: If you are lucky, you will only have to change that one gear; but, because of the spread of ratios you are changing, you may have to change the speedo drive gear [on the output shaft of the tranny] AND the speedo output gear to get the job done properly.
The ratio change you are making is 3.08/3.70 = .83 . Therefore, you need a speedo output gear that has only 83% of the number of teeth on your present output gear. Remove the speedo output gear housing from the tailshaft of the transmission, pull the gear off and count the teeth on it (it should also have the number of teeth cast or stamped into it somewhere). Multiply that number of teeth by .83; the result is the number of teeth on you gear that you need.
There are 3 different output shaft drive gears that are available. Each of those drive gears can accept several output gears and still be properly matched. But, each can be matched with different speedo output gears. So, depending on the speedo output gear you need, that selected gear may require that the internal gear be changed, also. If that is the case, then you will need to use a "Speedo Gear Calculator" program to determine the proper combination of speedo drive gear and speedo output gear that you need for your car.
Hopefully a speedo output gear with 17% fewer teeth will be the only change that is needed to get you going again.
Last edited by 7T1vette; May 6, 2013 at 03:43 PM.
Burning Brakes
Joined: Mar 2010
Posts: 803
Likes: 4
From: Christchurch, New Zealand
Just put a new speedo gear in myself, and the above explains quite a lot. I have 255/60/15 Cooper Cobras, a 3:08 rear, and a TH-400. My speedo was reading 5 mph too fast, so I needed a gear with one less tooth to reduce the reading by 5mph to make it correct. In the process, I found my TH-400 is from a 69! Anyhoo, replaced the 35 tooth gear with a 34, and she's all good to go.
Thread Starter
Racer
Joined: Jul 2007
Posts: 269
Likes: 0
From: WV
Alright sounds like when I get back to work on the car tomorrow I have some to look into. Has anyone ever used one of those ratio adapters that either slow down or speed up the cable with any luck? Starting to wonder if that may be better to try then dealing with trying to take a trans apart to switch the driven gear if that's what is needed..
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
One of those 'ratio adapter' is only necessary if you can't come up with the correct combination of available tranny drive and driven speedo gear sets. Some particular combinations are no longer available so that is when that "high dollar" mechanism might be required.
The vast majority of situations can be resolved by changing only the speedo driven gear (can be easily removed/exchanged from the transmission); some situations require changing both the drive gear (on the trans output shaft) and the driven gear.
Whatever vendor sells these gears (I know that Summitt sells them) has the calculator programs that can determine exactly what you need. Call them and discuss it. If you can't find someone to assist you [even though they want to sell you something
], send me a PM and I will help guide you through the process. It isn't difficult...just not something that most folks have done before.
The vast majority of situations can be resolved by changing only the speedo driven gear (can be easily removed/exchanged from the transmission); some situations require changing both the drive gear (on the trans output shaft) and the driven gear.
Whatever vendor sells these gears (I know that Summitt sells them) has the calculator programs that can determine exactly what you need. Call them and discuss it. If you can't find someone to assist you [even though they want to sell you something
], send me a PM and I will help guide you through the process. It isn't difficult...just not something that most folks have done before.
