Technical question about speedometer adapters

this in reference to mid 60's Stewart Warner aftermarket speedometers.

so, let's say a guy passes one of those speed limit signs with the "your speed" reader boards and it says 34 mph
you look down and your speedometer reads 30 mph
that's a difference of about 1.133 per mile

now the dumb question:

is that the ratio I'm looking for in the adapter description? assuming it refers to multiplying the cable rotation to spin the head faster.

anybody have a part number for an adapter that attaches to the speedometer end of the cable.. square cable with 5/8" x 18 thread nut

does the 777F have the 5/8x18 threads?

 

All the adapters I've seen fit on the transmission end of the cable. 7/8"

 

unfortunately I don't have room at that end
and I haven't found a source for driven or drive gears.. yet,
that may be my best route, I was hoping for an easy solution

 

You can probably get a cable made that fits the transmission and the other end fits the adapter so you can get the adapter away from the trans. I think any good speedometer shop that makes cables could do it.

 

something like this will let you put the adapter a ways from the transmission....

https://www.ebay.com/itm/281117083538

 

that's a lot more cost effective than the ballpark quote the local speedometer shop gave me, his estimate for two cables and adapter was near $700.00

 

I have no idea why, but speedometer things like that have always seemed way more expensive than what they're worth. It's not like they're incredibly complex and made of moon rocks.

 

5/8 to 7/8
https://www.ebay.co.uk/itm/360568281732

777f has 7/8 threads

 

What transmission do you have? Some have lots of different speedometer drive gears available for all the different rear ends. Changing the drive gear would be the cheapest fix.

 

I run a SW reducer for my Tach drive it’s the same square drive as the speedometer cable. Yes the ratio on the reducer/multiplier is what it gives you. For example I needed to cut the drive speed in half and the reducer says .500 right on it.

 

Seems to me that Rusty @e1956v from Speedometer Service in Colorado would be the person to ask about this. JMO

 

The cable drive on my trans comes out the side, and there isn’t a lot of room there to route the cable without hitting the exhaust. I have a ratio adapter that’s a 1:.8 reduction drive, but also turns the output by 90* in very little room.



This seems better than having to make a tight 90* bend in the cable.

 

@e1956v , are you able to help @Paul get his speedo reading correctly?

 

Look: https://www.ebay.com/itm/334951700849

 

I think there is a market for a speedometer adaptor which works like a miniature scooter CVT minus the centrifugal bits. That way separate tooling isn't required for each ratio.

 

I can build ratio adapters but not 90 degree adapters. I can also make up the cable Squirrel posted but not at that price, the 7/8-18 male fitting is $15.00 my cost so I’m not touching a $23.00 price point.
All domestic ratio adapters are 7/8-18 thread, as for X38 and Ned’s comment on price the set up to cover 100% ratio change both plus or minus is not cheap nor are the new parts. The pic of the book shows all of the possible ratios.
My price on new adapters is $110.00 and I’m not getting fat at that price.
If you know the ratio you need you can try to find a GM 90 degree adapter in that ratio if you look long enough.

 

What kind of transmission are you using? Here is some info for GM.

 

Since he can't find the gears, it's pretty darn likely that it ain't no TH350 he's dealing with.

 

the speedometer drive portion of the transmission in question is from a 1958 Oldsmobile Synchromesh Selector Shift,
differential is early 60's Olds 9.3 with 3.42 gears, tires are 29" tall..

I'm probably finding dead ends on the search for driven gears by being too specific on the transmission, there must be other 50's GM cars and trucks that used interchangeable gear sets.

 

There wasn't so much interchangeability back then....

but pictures and dimensions of the gears you have will go a long ways towards others being able to help you.

 

That is steep. I used to get my adapters for my mud trucks from the transmission shop. They had them that went on the tranny or the speedo. I tried the tranny ones and they proved to be a problem as we actually drove off road I (a lot). The speedo end adapters ran about 30% more back then I am not sure why.

700 is a little steep. Glad that Jim (@squirrel ) could help you out.

 

Is there any adjustability of the spring tension inside the SW speedometer? Like, can you tighten up the spring tension a bit so it would read slower?

The speedo in my '57 Chevy started shrieking really loud and then broke completely, so I had to take it apart and learn all about speedometers to figure out how to fix it. There is a really light clock spring in there, keeping tension on a drum that is attached to the needle, and a magnet that spins inside that drum, like a little magnetic torque converter. On the Chevy, there is some adjustment of how much tension you can put on that spring. A bit more tension would keep the needle slower, since the spinning magnet needs to overcome that spring to move the needle.

No idea if the SW speedo does that, or if it would be enough to make up the difference, just a thought. The inside of a speedometer is a scary place, like the back of a watch. I did manage to get my '57 speedo working again after it broke but it isn't super accurate either, so take my advice with a grain of salt.

 

driven gear is 21 tooth, thread into transmission measures 0.930, presumably 0.9375 for 15/16" with 16 tpi, overall length just a hair over 3"

 

I don't have drive gears, but I do have a drawer of reducers and adapters things. And BOXES of speedo cables... if I can help, I'd be happy to look. I'll start with seeing if I have anything I think would work.

 

I believe Buick, Olds, and Pontiac all used that transmission.

 

my Olds Parts book shows only two listings that include '58 standard transmission driven gears
20 tooth and 21 tooth
21/20 = 1.05
guess I'm on the lookout for a 20 tooth driven gear of the above dimensions..
it's a place to start

 

The trans fitting in your picture above looked familiar, so I pulled the one out of the extra Hydro I have out back. The fitting looks the same but the gear is totally different, smaller diameter with a different shaft. Dang.

 

The old SM420 truck transmission also has a very similar sleeve, but different gear shape.

 

The fellas have you pretty well covered on the adjusting('fixing') the speedometer, but I would like to add a small note.

Yeah, no.
Most of those 'your speed' signs are not a reliable source of actual speed. They can be a good indicator of your general speed. Depending on location, road condition, and who programed/setup the sign, 'your speed' results may be woefully inaccurate.

Two options to check your speed.
Bring it to a speedometer/headlight shop adjustment shop.
Or.
I would suggest finding a stretch of road with well placed mile markers and getting a 'navigator' with a stop watch. Hold a steady speed and time it.
60MPH over a mile should be a minute per mile marker. Hardest part of this would be doing it without speed interference from traffic.