Speedometer

[57halfton]

The speedometer in my 57 Chevy pickup hasn't worked for sometime now, i have checked the cable and the speedo gear in the trans and everything looked ok so today I swapped out the speedo cluster for another one I had.
After I got everything back together I drove it around the block and still nothing worked so I took the cable off of trans and put a drill motor on it and the speedo went up too about 45 mph, I then took the speedo gear out of the trans and it looked ok so I put everything back together again and drove around the block again this time the odometer seams to be working right but the speedo is reading slow, like 8 mph and 30 mph according to a sign that tells you how fast you are going when you go past it.
Anyone got any ideas? All the running gear (trans, speedo gear, and the rearend gear raito is all the same as it was before the old speedometer quit working.
Thanks , Gene

[NEWFISHER]

Dramatic tire size change? Lube the cable so it doesn't jump mabey take the cable off the new speedo and screw it to the old one and hang it from the dash. Drive again and see if there iss a change. Reinspect the speedo gear? That a weird one for sure
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[mr48chev]

I agree with pulling out the cable and cleaning and lubing it. About 50 years ago I was taught to use white lithium grease to lube those and it's worked well for me ever since.

[57halfton]

speedo cable has been cleaned and lubed and no change in rear tire size. I am thinking it may have something to do with the small speedo gear, I'll have to look at a new one and compare it to the old one.
Thanks , Gene

[gregw98]

I am having the same issues with a 350 tranny. I haven't yet figured out which gears I need to get for it.

[57halfton]

I am thinking of replacing both gears in my 350 turbo. I think they cost about twenty or thirty bucks and it dosen't look to hard to change them, just get the same color gears as the ones you have now.
Gene

[gregw98]

Before you lay any money out for new gears, check out these two sites.

Almost everything you need to know - - -and more

http://www.cadvision.com/blanchas/54...eedometer.html

All you need to know

http://www.tciauto.com/tc/speedo-info

[butchp]

This is what I did, remove the speedo again and put some really good lube in the back where the cable attaches, there is a leather item in there that gets dry. This made mine start working correctly.

[sqrlnts]

Originally Posted by gregw98

Before you lay any money out for new gears, check out these two sites.

Almost everything you need to know - - -and more

http://www.cadvision.com/blanchas/54...eedometer.html

All you need to know

http://www.tciauto.com/tc/speedo-info

Your first link is dead can you recheck it because I am gathering information on this subject.
Here are some rebuild instructions.

[gregw98]

http://www.cadvision.com/blanchas/54...eedometer.html

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

And another method from a friend.

I calibrate my own speedometers by the following method:

1) Make a mark on the ground and one on the rear tire.

2) Roll forward until the mark has gone around once.

3) Make a second mark on the ground that lines up with the mark on the tire.

4) Measure the distance between the two marks on the ground. (mine comes out to 80.5 inches)

5) Divide 63,360 (which is the number of inches in one mile) by the number of inches the van rolled forward in step 4. This will tell you in very accurate terms how many times the rear tires turn to travel one mile. (I come up with 787 revolutions per mile for mine)

6) Multiply the above number by rear axle ratio. This will tell you how many times the driveshaft turns per mile. (787*4.88=3840 for mine - I'm running 4.88's because I have an overdrive tranny)

7) The speedometer head needs to turn exactly 1000 revolutions per mile to read accurately, so you need to divide the result in step 6 by 1000. (3840/1000=3.84 for mine)

8) Take off the tailshaft housing and count the number of teeth on the drive gear, which the one attached to the output shaft. (hint - it's easier to count the starts of the teeth on the end of the gear rather than the side)

9) Count the number of teeth on the driven gear. Divide the two. The result needs to equal the number you came up with in step 7. (I have a 9 tooth drive gear and a 35 tooth driven gear which results in calibration that's really close - 35/9=3.88) If it doesn't, then go to the junkyard or the local tranny shop and get a set of gears that will divide out right. Some transmissions use small gears and some large, so keep that in mind when you shop for new gears. It doesn't matter if you have an overdrive, because the output shaft always turns the same number of times per mile.

And another:

http://www.tciauto.com/Products/Comp...eter_gears.asp