Speedometer calibration with a 700R4 Trans

[Bruce88]

Speedometer calibration with a 700R4 Trans

The concept for most transmissions is the same getting the right drive gear to the right driven gear so the speedometer reads the correct speed. Other transmissions use different gears, housings, and retainers even the electric transmitter models. So this “how to” will only deal with the 700R4 mechanical speedometer set up (a popular transmission swap for the 67 – 72 Chevy trucks)

I’ve borrowed a few part pictures from PATC Raptor Transmissions web site to help in visualizing the process. I’ve used them for parts and found good service and reasonable prices http://www.transmissioncenter.net/sp...n_______va.htm . There are many places you can find the parts including the junk yard.

The most expensive part to acquire or find is the “Driven Gear Housing” there are two different ones depending on what gears are used. Driven Gear Housings run approximitly $35.00 used to $70.00 new.

When ever you’re working on an automatic transmission I recommend using new O-rings and seals lubed with trans fluid and always make sure that the sealing surfaces are clean with no damage before assembly. (New seals are cheep insurance to prevent leaks later on)

Hear is a picture of some of the parts that you will be dealing with



There are a few things that you will need to know before you start gathering your parts.
1 Tire Diameter – I will be using 27 inch tire diameter for my installation
2 Differential gear ratio – I will be using 3.42 ratio for my installation
3 Drive gear teeth – I will be using a Gray 15 tooth drive gear (note my
transmission came with a Red 17 tooth drive gear)

You may not have to change the drive gear as I am, all the driven gears work with the color coded drive gears shown above. I already had a driven gear housing that works with the 34 to 39 driven gears and it was cheaper to replace the drive gear and new seals than get the other housing. After putting in tire diameter, differential gear ratio and drive gear teeth that you have in the calculator, you might not be able to find a driven gear tooth count that will work with your drive gear. If this is the case you can try deferent drive gears in the calculator until you find one that works.

Hear is a picture of the drivers side of the transmission and the driven gear housing removed to find out what drive gear is installed.



There are many speedometer calibration programs that can be found on the web. They all work in a similar manner. You plug in the information you gathered and it calculates what your driven gear should be. I took some screen shots of the calculator that I used http://www.bgsoflex.com/speedo1.html shown below.



After you have gathered all the parts that you need its time for the installation.

If you need to change the drive gear you will need to remove the drive shaft then the transmission rear housing. On some 700R4 transmissions you will find a cylinder/cup with seals just behind the drive gear it will slip off the shaft so that you can change the drive gear. When reassembling reinstall the cylinder/cup if so equipped.

Hear are some pictures showing changing the drive gear



Now to the final steps installing the driven gear and housing. I think the picture below will cover all the information required



If anyone sees where clarification or additional information is needed please feel free to contribute.

[68SHORTBOX]

The torque for the tail shaft housing bolts is 26 ft lbs. Very nice write up and a great link.

[67ChevyRedneck]

Originally Posted by woodridge

Good thread. I didn't know about the speedo rpm's.

I didn't either. So 1,000 if for all 67-72 stock speedos? Where did you find that info?

[Bruce88]

Originally Posted by 67ChevyRedneck

I didn't either. So 1,000 if for all 67-72 stock speedos? Where did you find that info?

I wish I had a answer for you and a manual to point to that has the specification data but there is none that I have available to me. I can say that by using the 1000 revolutions per mile in your calculations your speedometer will read correctly for a 67-72 Chevy truck. Most cars and trucks from 1965 and up use the 1000 revolutions per mile on mechanical speedometers.

[nightmare]

Originally Posted by 67ChevyRedneck

I didn't either. So 1,000 if for all 67-72 stock speedos? Where did you find that info?

Originally Posted by Bruce88

I wish I had a answer for you and a manual to point to that has the specification data but there is none that I have available to me. I can say that by using the 1000 revolutions per mile in your calculations your speedometer will read correctly for a 67-72 Chevy truck. Most cars and trucks from 1965 and up use the 1000 revolutions per mile on mechanical speedometers.

IIRC, they are stamped in ink on the backside of the speedo.

[nightmare]

I didn't see any part number's listed, so I found some that should be posted too:

[sleepertruck72]

I need to do that, but my tranny is in the truck already. Does it need to drained of tranny fluid ? Ive been driving it that way for awhile now, so I could live with it I guess.

GREAT write up by the way !!!

[6772owner]

If you have to change the drive gear (on the tailshaft) you have to remove the tailshaft housing so its best to drain it in my opinion. I have changed the driven gear several times without draining the trans, I just jack the rear up to put the fluid towards the front of the trans. You can also pitch it a little to the passenger side, this will give you minimal fluid lose, just top off the trans when finished.