The Stovebolt Site Forums The Parking Lot Big Bolts Speedometer adapter for two-speed rear axle...

Howdy all,

I'm curious about these. My truck speedo doesn't work, and I'm hunting for one. In the meantime, I've learned about an adapter that some big bolts have on their speedos to compensate for the two-speed rear axle. My truck does not appear to have one bolted to the back of the speedo.

Anyone know anything about these?

Thank you.

They are small two speed transmissions, with the same % difference in ratios as the rear axle has.

If not mounted directly behind the gauge they can be anywhere in the speedometer cable, second most common place is on the firewall.

If behind the dash it's likely mechanically shifted.
If you ave an electric two speed then the adapter could be electrically shifted. Not sure if they ever shifted the adapter with vacuum, but probably?

Grigg

Thanks for the information. So far, I know the cable went straight from the tranny to the speedo when I disconnected it to diagnose the problem, but that doesn't mean the adapter wasn't bypassed in the past and it's still under the dash somewhere. I'll have to take a look for something I don't recognize. I suppose it's possible that someone in the past removed it because they thought it was the reason the speedo wasn't working and then just left it off when it didn't fix the problem. Or maybe it really went bad and was removed.

Is it possible that the adapter was never there? Or were these standard equipment on all trucks with a two-speed axle?

The two-speed axle on my truck is vacuum shifted, what would I look for as a possible connection to the speedometer adapter? Perhaps I could follow it to the location where the adapter is or used to be. I've clearly seen the vacuum switch bolted to the side of the tranny..in fact, I temporarily removed it when I changed out the clutch and had to remove the tranny.

I understand all two speed rear axle trucks have the adapter. It is strictly mechanical no vac invloved.

If the speed-o-meter cable went stright into the speed-o-meter gage, then the adapter was missing. It screws right into the back of the speed-o-meter gage where you found the cable attached. Imagine where the cable meets gage, the adapter fits there between the cable and gage rear...

If there was no adapter inplace, it is likely you do not have the piece of hard wire that attaches to the two speed axle switch/dial that mechanicly changes the rato in the adapter on the back of the speed-o-meter. It is just mechanical flip of a lever on the adapter. I'll try to find you a picture.

Virginiafiretruck, he may not have the same setup in his 59 as we do in our AD trucks.
Where is your two speed axle switch mounted? Do you have any pictures of it? The adapter is likely not far away from this vacuum valve.
A truck with a factory installed two speed would have come with an adapter, did your truck come new with a two speed?

Grigg

I didn't pick up that he had a 1959... My bust.

The 2-speed rear speedometer adaptors for the most part on GM rears are all the same. The ratio multiplier in the rears is a fixed 1.39. If you take the Hi range ratio and multiply it by 1.39 you get the Low range ratio. With that said, there are only one or two adaptors that fit almost all applications, so if you see one in the junk yard grab it even if you don't know what the rear ratio is, chances are it will work.

If you have an Eaton 2-speed rear (these are all electric) the ratio multiplyer is 1.36 and can be found on almost any brand truck...I got mine out of a 60's International.

All GM rears were vacumn shift (at least during the TF years). They have a cable operated vacumn switch that is mounted on the drivers side of the transmission at the base of the shifter. The switch has 4 ports, main vacumn supply, Hi range to the rear, Lo range to the rear and a small port to the speedometer adaptor. The adaptor is mounted right on the back of the speedometer. It has a small vacumn diaphram on it that shifts it from 1:1 ratio (Hi range) to 1:1.39 ratio (Lo range). It's hard to find a good vacumn adaptor, seems like they all have rotten diaphrams and leak.

I have the parts laying around and can take pictures if someone needs to see what the look like.

Hope this helps.

Mike B

Thanks for all the info so far. If anyone has anything to add, you can be sure I'll relish the info.

I do believe my truck came from the factory with the two-speed rear axle. It is a 1959 model, sorry I didn't mention that outright from the beginning, it might have saved some confusion. According to the registration papers that came with the truck, that date every year back to around 1965, the truck was once a tanker for a fuel company out of Durango, Colorado (It was converted to a flatbed sometime in the 1980'). So hauling that kind of weight, it probably had the two-speed. (It's original GVW was 19,500, thankfully the New Mexico MVD let me "de-rate" it to 10,000 lbs, which saved me a lot of moolah on the registration fee!)

It does have the mechanically operated vacuum switch on the left (driver's) side of the tranny. I'll have to follow the vacuum lines and see where they go. The rear axle hasn't shifted since I got the truck a couple months ago, and it could be that the line that ran to the speedo adapter is just hanging there, leaking vacuum, which could account for the non-operation of the rear axle.

There definately is no adapter on the back of the speedo, and the speedo is not working anyways, so I'll be on the hunt for a whole unit, complete with speedo adapter.

Hi,
I've parted out a late 40's-early 50's GMC 300 that
came with a 2-speed rear. The vacuum switch was mounted to
the firewall, with the round handle mounted under
the dash. All the vacuum piping was in steel flare nut
tubing, so it appeared to be factory, in fact
the owners manual showed the exact control the truck
had. The speedo adapter attached to the back of
the speedo, and was also vacuum run.

I sold the control to a member here this summer, but he
did not want the speedo adapter. It is obvious by looking
at it that the vacuum bellows needs replaced, but
otherwise looks to be in working shape.

If you want it, you can have it for the cost of shipping.

Thanks,
Brian

Here is a clever idea for anyone who is having problems repairing a vacuum actuated two speed, speedometer synchronizer with a blown diaphragm. Here is what I did I went looking at 70s and 80s emissions carburetors they have so many little vacuum actuators on them, I found one that I thought would be easy to adapt and made a little bracket and bingo better than new.

Very clever.... I will make a mental note of that.

That would be great. I still need to find a working speedometer as well, but I've got a couple leads. (All the gauges on the truck now work except for the speedo).

Doe anyone have any idea if an adapter from a late 40's, early 50's truck will work on the back of a 1959 speedo? Is the attachment/screw pattern the same?

I do have the whole thing speedo and adapter from a 57 6400 the cluster is in good shape lens is clear and the bezel has minor pitting in the chrome. $100 plus what ever it would cost to ship.
Fellow bolters may weigh in here I do want to offer it at a fair price and I don't really know????
I know that we have a classified section but this wasn't really for sale he needs one so I am offering.

Thank you for the offer, HevyHauler. I really have no idea what a fair price for the speedo and adapter would be. I bid on a speedo unit on Ebay this morning, with adapter, and won it for $26 plus $15 shipping. Ebay pricing can vary widely depending on how something is listed and the audience the wording attracts. For example, I bought a late 1940's or early 50's 20 foot travel trailer in fairly good condition about two months ago, for $115.49 on Ebay. It had been sitting in the same spot in Arizona, buried in pine needles, for about 40 years. The Arizona license plate expired in 1965. I aired up the tires that were on it, split rims, inner tubes and all, and pulled the thing about 250 miles to my backcountry camp in New Mexico, where it will likely never move again. The tires are still holding air! It turned out to be a real jewel, and a steal at that price.

I also got ripped off on a "set" of tail light lenses for a 1960's Jeep Wagoneer that were supposed to be "NOS", and I paid a premium for, but turned out to be from two different decades, one still in the original bag, the other not. The seller basically explained it by saying "whoops, must have shipped one of the wrong ones", and then I never heard from him again.

In the photo of the speedo I won one Ebay, the bezel looks either rusty or simply very dirty, but the one on my truck is very good, so I wasn't too worried about that. It's supposed to be mechanically sound, and the seller has good "feedback", and lots of it, which is basically the sellers reputation. So I shall see what I get in a few days. I'm only going to use what I need, which is the actual speedo mechanism and the adapter, and the rest will go in a box for spares, I guess. If it turns out to be mechanically "as described", then I suppose I got a pretty good deal.