speed gauge not accurate

SlyMart

SlyMart

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Hi there

I recently posted that my speed gauge is not accurate. I finally find out why I am having this issue. The previous owner upgraded the differential from the factory 3:55 to 3:73. The speed cluster cable was recently replaced but the issue with the speed gauge not being accurate is there. A mechanic told me to buy a piece of plastic for the cluster cable in order to resolve this issue. What do I really need to fix this issue?
 
Hi SM..
You need a new gear-set in the side of the transmission where the speedometer cables fastens.
There are different colour gearsets for different rear end gears.
Do you have a 4-speed or automatic tranny?
Here's a link to a site with pictures.
http://www.transmissioncenter.net/speedometer_calibration_______va.htm

The Assembly Manual has some sketches and a gear table for manual transmissions.
Page #422(12J-5) page #423(12J-6)

You should be able to find a gear that works for your set-up.
Graham
 

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My '75 coupe 4sp turns 3900rpm @ 80MPH with 3:55s
The ZZ4 motor loves it.
Surprisingly I get good gas mileage on trips.
Sometimes I wish I had an overdrive but this is fun.
I can't imagine 3:73..it would be 'funner' but like Brian says, not so much on trips.
Can you tell us your rpm at a given speed..?

graham
 
My rpm gauge and the speed is not accurate. so if I am going at 100 km/h the rpm is at 4500 rpm...

I called my local part store and he has no clue what part I need to buy to fix this issue..

I am thinking maybe I should go back to 3:55 since I don't really care how fast she goes... This car was purchase just for show...
 
To try and solve the speedometer you need to decrease your driven gear from the 3:55 stock 22 tooth to a 20 tooth. This is the gear that your speedometer cable plugs into. NAPA should carry it, just ask for a 20 tooth DRIVEN gear for a TH 350.
 
No receipts of the work done by the previous owner? You'll need to get the car's rear end up in the air, put her in neutral, and rotate those tires to get a better idea of exactly what you have in there. The 3:73's weren't available in '78, they offered a 3:70; which is why I offered up the 20 tooth driven gear - that's what would have been put in by the factory. As Graham has stated, the tach seems to be sitting probably correct for cruising speed, to verify against a GPS would probably be the easiest way to go about it.

If going back to 3:55's is in the cards, I can't emphasize enough that you get in touch with a shop that KNOWS these diffs and how to set them up properly. Definitely a job you only want to pay to do once.
 
Brian ..good call on the GPS to verify actual speed. simple/quick.
SM.. Connect an aftermarket tach to your car to verify rpm.
I have a 4 speed in my '75 but in top gear it should be the same as your auto in top gear.
I turn 3900rpm at approx 125kph(80mph), so approx 3100 @ 100kph(62mph)..?
My car isn't insured right now otherwise I would take it out and snap a gauge picture for you.

If you do have 3:70 rear gears that is OK.
I don't think I would spend the money changing it back to 3:55.
I would look at a very popular transmission swap to a 700R4 automatic with overdrive.
This will give you the benefits of the tall gears and with the overdrive on the highway, great fuel economy.
This may or may not be correct if your planning to keep the car all stock or your special collector insurance allows this swap.

graham
 
I have that exact set up in my '79 (TH350/ 3:55), although I run a stock L48 - no difference there though, as the gearing through the drivetrain is what we're discussing. Not sure about Graham's final drive ratio, but the '78 and '79 four speeds and TH350 3 speeds both have the same final 1:1 ratio, so at least you can compare apples to apples in that case. At 100 kmph, I sit at about 3000 - 3100 rpms. My tires are 255/60R/15 on all four corners.

I have a 2004R that has been in my shed for about a year and a half now. A slightly easier swap than the 700, as the driveshaft doesn't have to be cut. I'll get around to it sooner or later.
 
You're not really sure what ring and pinion you actually have, so right now, it's a mute point. Get the rear on jack stands, car in neutral and spin the rear wheels one complete rotation. Count the number of rotations of the driveshaft to see the rear end ratio. Once known, verify the correct driven gear, and replace if necessary. Then you will have known variables with which to make an educated decision whether or not to make any major changes. This will cost you less than $20 and an hour of your time.
 
The reason I was asking for the difference between the 3:55 and 3:70 driving at 100 km /h is for me to see the difference in term of RPM level. If I look at both then I can decide if it's worth staying at 3:70 or go back to 3:55. I will jack the car up this weekend and check the actual gear in my car :)

 

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