Thread: Need Help Speedo off like 20mPH or more

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Can anyone tell me how i can get my speedo to be more inline with what is supose to be. I have a 1971 Nova, 350 th tranny. I just replace the dash bezel and . the speedo cable was unhooked so i hooked it back up and when you go down the road like 60 it reads more like 90.
Is there a gear in the tranny that can be changed or why would it be off so bad??????

Any help !!!!!!!

Hi Novacane,

I would start with the simple questions first like,

What is the size of your rear tires and wheels?
* I've noticed the biggest change in speedometer accuracy has usually been do to the size of the rear tires and wheels.
Is the tranny the original one to the car?
If it is, has it been rebuilt?

This has obviously been an issue in your car since the speedo cable was previously disconnected, but it may be something simple.

Good luck,

Greg

The tires in the rear are 235/60/15 ,Yes i believe they should be 14 inch, but does that make 30 mph difference?
I am not 100 % sure the trans is original, I know the motor is a 73 built, The trans has had a b&M shift kit with a rachet shifter ,3200 stall,rear end is 4:11 posi

So what are the simple steps????? Or could it be a bunch of problems?

Thanks Greg

Here's a good site to answer your questions.

http://www.transmissioncenter.net/sp...n_______va.htm

And another "solution".

http://www.atrol.com/cablex.htm

Thanks Pat, This is a good direction for me to start. Didnt check out the cable x yet , scared to see the price tag. But that would most likely be the easiest.

Greg...I typed in "Speedometer Calibration" on the Google search engine and came up with a lot of interesting sites. Here is one that allows you to plug in info about your current settings and it will give you "actual" MPH.

http://www.csgnetwork.com/speedocalibcalc.html

Pat , Thats pretty slick, But there has to be other factors here in this application, because if i but in the stock and the the tire size thats there now its only off by about 1 1/2 miles per hour. Were talking 20 to 30 here. Maybe im just flying lol . No its really off that far. Thanks for the info though.
Dave

Originally posted by Pat Monaco
Greg...I typed in "Speedometer Calibration" on the Google search engine and came up with a lot of interesting sites. Here is one that allows you to plug in info about your current settings and it will give you "actual" MPH.

http://www.csgnetwork.com/speedocalibcalc.html

Wow, that is a great calibration table! I've seen others like that before. The most interesting thing I find about it is the difference that the actual height of the tire makes with regards to "Actual" spped. You can basically do the same thing on a portable dyno trailer. Of course, not everybody has one of those in their shop!

Thanks for the link!

Greg

The 4.11 gears explain it. The tranny has a gear for, like, 2.73 gears. That'll put the speedo off that much.

All, TCI Automotive provides the technical information you are looking for and a group of calculators to help you determine which speedometer gears are needed for your application.

In the 350 application you will need a drive gear, a driven gear, Drive gear clip, and a driven gear housing (which you should already have). All this should be less that $30 new from TCI. Check out the following link.

go to http://www.tciauto.com/tech_info/speedo_gears.htm


jc

Thanks JC usefull information

divide 20168 by the tire diam. in inchs this will give you revolutions per mile then use this formula # drive gear teeth X axle ratio X tire rev per mile. _______________________________________= #driven teeth 1001 the drive gear is on the out put shaft driven gear is in the apaptor housing

It should have read divide drive teeth X axle ratioX tire rev. by 1001 to give you driven # teeth

Another thing is on an older car like yours- and mine- compared to a new digital or other modern car speedo, it's easy to get at by comparision and speedo shops charge a lot less. Decent shop can make changes and calibrate it for less than you probably think. Hey estimates are free.....