57 stock speedo 700r4 speedo gears???

[zenchal26]

Has anyone out there already have this setup in there truck? What speedo gears did you use to get the gauge to be accurate? I know rear end and tire size are going to make a difference trying to get a good starting point so I don't spend a fortune in gears.

[OrrieG]

Rear gearing and tire size will be a factor too. Once you get the truck running get a gps and compare actual speed to what the speedo is reading (so much easier than the old method of driving miles at a constants speed to get a time). Calculate the difference up or down.

Pull yours and see what color it is. Use the color chart in this thread http://www.thirdgen.org/forums/suspe...oth-count.html
to determine which color you need to get the same percentage change.

If you want to play hypothetical tire size and rear gear ratios, go to this thread and use the calculator down the page to play with the ratios.
http://www.73-87.com/7387garage/drivetrain/speedo.htm

[zenchal26]

thx for the links. already gps the truck its showing about 20-25 mph faster than truck is going

[56 Woody]

Are you using the original speedo, They have new that look old but are GPS activated. You can look around and find some if that helps.

[zenchal26]

original speedo

[zenchal26]

anyone know the stock rear end gear ratio for 57 3100 6cyl? no tag on mine

[mr48chev]

There are a couple of ways you can correct the speedometer reading.

First you want an accurate reading so you can figure the percentage that it is off an which way. As it is reading fast you need to use a smaller drive gear and larger driven gear to slow it down a bit.
You also need to figure out what gears you have now so you can figure out what gears you need to change the reading.

This shows available gears for the 700R4 and the housings you use depending on tooth count.
http://www.tciauto.com/tc/speedometer-gear-calculator/
Scroll down to the right trans.

There is another way to do it and that is by using a small gear box that attaches to the trans and with the correct gears inside changes the ratio in and out and corrects the speedometer. They use those on Semi trucks and 4x4 trucks a lot and I found one on my 77 1 ton flatbed the other night when I was looking at the cable to figure out why the speedometer wasn't working.

[mr48chev]

I slid under the 77 with my camera tonight and took this photo of the speedometer correction box that fastens to the trans and with the right gears inside corrects the speedometer.


From the back side And yuppes. that is one greasy transmission. That is a work truck that only leaves the yard when it has a job to do.


I know you can find them at Semi truck shops for about 60 bucks and they are available though some 4x4 shops to compensate for larger tires. When I worked for Ryder truck rental we used them on the trucks to correct the speedometer and odometer readings.

I still can't find my go to speedometer calibration and gears page that I have dug up for folks for years.

[bosco]

Originally Posted by zenchal26

anyone know the stock rear end gear ratio for 57 3100 6cyl? no tag on mine

I believe the stock rear gear ratio is 3.90

[OrrieG]

Originally Posted by zenchal26

anyone know the stock rear end gear ratio for 57 3100 6cyl? no tag on mine

http://chevy.oldcarmanualproject.com...esto/56139.htm

[1project2many]

Yep, 3.90 in most cases. Some may have had 4.10's. You should confirm the ratio though rather than guessing the rear has never been changed.

How to find out your diff gear ratio - YouTube

Here's neat little calculator:
http://tri-5.chevyrides.com/speedo.php

[Coupeguy2001]

There is a transmission shop called MONSTER TRANSMISSIONS. I bought one of their adapters that had a vss and a speedo cable attachment and when i use the tuned port, I will still use the original speedo. The 700R4 and the Doug Nash 4+3 trans uses the same speedo adapter.
Mine is a Doug Nash.
I have 7 forward speeds. It's like a gear splitter in 2nd, 3rd, 4th.I still can go any combination of 2nd, 3rd, 4th, and overdrive or no overdrive in each gear.
The speedo gear I used was the green gear. different colors denote different gear teeth.
my speedo is about 5 mph off at 75..... I have a 3.48 rear with 27 inch tall tires.
Taller tires would make the speedo more accurate.
In overdrive in 4th, the truck will go 75 mph @ 1850 RPM

The trans gear you will need is one from an 89 Suburban 4wd with 4.11:1 gears with a 700R4.You may have to trade out the ring gear on the trans output shaft.
The tailshaft housing comes off easy, there is a clip on the tail shaft that holds the shaft gear in place, and you just use a slide hammer gently with a J hook.
gently tap the new ring gear in place using a piece of pipe that goes over the shaft and put the housing back on.
You will need a new gasket for the tailshaft housing, and some use an o ring and some use a gasket, depending on the trans you have

[mr48chev]

I had to pull the speedometer correction box out of the 77 to clean up the threads on the output side so the new cable would screw down tight.





The gears inside. I cleaned it up and put new grease in it.